docs(adr): ADR-045 + RTM rows for the #827 grouping modes

Record the three #827 forks (client-side regroup transport, computed letter→event
link reusing timeline_event_documents, filter-then-group composition with #780)
as ADR-045, trace REQ-001..019 (+005b) into the RTM as Done, and list the new
timeline components in the frontend domain inventory.

Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.specify/rtm.md

@@ -2,7 +2,7 @@
 
 > Living document. One row per `REQ-NNN` across all in-flight and shipped features. The spec
 > itself lives in the **Gitea issue** (issue-only — there is no committed `spec.md`); this
-> matrix is the part of the spec that *is* committed: it links each requirement to its issue,
+> matrix is the part of the spec that _is_ committed: it links each requirement to its issue,
 > the code that implements it, and the test(s) that prove it — so any requirement traces end
 > to end, and any orphan (a requirement with no test) is visible on `main`.
 
@@ -25,16 +25,16 @@
 ## Matrix
 
 | REQ-ID  | Requirement Summary                                                    | Issue    | Feature                            | Implementation File(s)                            | Test(s)                                                                                                                                                                                                                    | Status  |
-|---|---|---|---|---|---|---|
+| ------- | ---------------------------------------------------------------------- | -------- | ---------------------------------- | ------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------- |
-| REQ-001 | Store avatar at `avatars/{userId}`, overwrite | #example | profile-picture-upload (_example) | `UserService` (planned) | `UserServiceAvatarTest#storesUnderUserKey`, `#replaceLeavesNoOrphan` | Planned |
+| REQ-001 | Store avatar at `avatars/{userId}`, overwrite                          | #example | profile-picture-upload (\_example) | `UserService` (planned)                           | `UserServiceAvatarTest#storesUnderUserKey`, `#replaceLeavesNoOrphan`                                                                                                                                                       | Planned |
-| REQ-002 | Upload self avatar → 200 + avatarUrl | #example | profile-picture-upload (_example) | `UserAvatarController`, `UserService` (planned) | `UserAvatarControllerTest#uploadReturnsAvatarUrl` | Planned |
+| REQ-002 | Upload self avatar → 200 + avatarUrl                                   | #example | profile-picture-upload (\_example) | `UserAvatarController`, `UserService` (planned)   | `UserAvatarControllerTest#uploadReturnsAvatarUrl`                                                                                                                                                                          | Planned |
-| REQ-003 | Delete self avatar → avatarUrl null | #example | profile-picture-upload (_example) | `UserAvatarController` (planned) | `UserAvatarControllerTest#deleteClearsAvatar` | Planned |
+| REQ-003 | Delete self avatar → avatarUrl null                                    | #example | profile-picture-upload (\_example) | `UserAvatarController` (planned)                  | `UserAvatarControllerTest#deleteClearsAvatar`                                                                                                                                                                              | Planned |
-| REQ-004 | No avatar → null + initials placeholder | #example | profile-picture-upload (_example) | `UserProfileView`, avatar component (planned) | `avatar-placeholder.svelte.spec.ts` | Planned |
+| REQ-004 | No avatar → null + initials placeholder                                | #example | profile-picture-upload (\_example) | `UserProfileView`, avatar component (planned)     | `avatar-placeholder.svelte.spec.ts`                                                                                                                                                                                        | Planned |
-| REQ-005 | ADMIN_USER may delete others' avatar | #example | profile-picture-upload (_example) | `UserAvatarController` (planned) | `UserAvatarControllerTest#adminDeletesOthersAvatar` | Planned |
+| REQ-005 | ADMIN_USER may delete others' avatar                                   | #example | profile-picture-upload (\_example) | `UserAvatarController` (planned)                  | `UserAvatarControllerTest#adminDeletesOthersAvatar`                                                                                                                                                                        | Planned |
-| REQ-006 | Unauthenticated → 401, store nothing | #example | profile-picture-upload (_example) | `SecurityConfig`, controller (planned) | `UserAvatarControllerTest#unauthenticatedReturns401` | Planned |
+| REQ-006 | Unauthenticated → 401, store nothing                                   | #example | profile-picture-upload (\_example) | `SecurityConfig`, controller (planned)            | `UserAvatarControllerTest#unauthenticatedReturns401`                                                                                                                                                                       | Planned |
-| REQ-007 | Non-image → 400 UNSUPPORTED_FILE_TYPE | #example | profile-picture-upload (_example) | `UserService` (planned) | `UserAvatarControllerTest#rejectsNonImage` | Planned |
+| REQ-007 | Non-image → 400 UNSUPPORTED_FILE_TYPE                                  | #example | profile-picture-upload (\_example) | `UserService` (planned)                           | `UserAvatarControllerTest#rejectsNonImage`                                                                                                                                                                                 | Planned |
-| REQ-008 | Over 2 MB → 400 AVATAR_TOO_LARGE | #example | profile-picture-upload (_example) | `UserService`, `ErrorCode` (planned) | `UserAvatarControllerTest#rejectsOversize` | Planned |
+| REQ-008 | Over 2 MB → 400 AVATAR_TOO_LARGE                                       | #example | profile-picture-upload (\_example) | `UserService`, `ErrorCode` (planned)              | `UserAvatarControllerTest#rejectsOversize`                                                                                                                                                                                 | Planned |
-| REQ-009 | Non-admin on others → 403 FORBIDDEN | #example | profile-picture-upload (_example) | `UserAvatarController` (planned) | `UserAvatarControllerTest#nonAdminForbiddenOnOthers` | Planned |
+| REQ-009 | Non-admin on others → 403 FORBIDDEN                                    | #example | profile-picture-upload (\_example) | `UserAvatarController` (planned)                  | `UserAvatarControllerTest#nonAdminForbiddenOnOthers`                                                                                                                                                                       | Planned |
 | REQ-001 | Render dated entry via shared `formatDocumentDate` (de/en/es)          | #778     | timeline-date-label                | `frontend/src/lib/timeline/dateLabel.ts`          | `dateLabel.spec.ts` › `renders a DAY date localized in German`, `renders a SEASON date with the German season word`, `delegates a same-year RANGE to formatDocumentDate`                                                   | Done    |
 | REQ-002 | Non-UNKNOWN + non-empty date → shared label, `raw=null`, `getLocale()` | #778     | timeline-date-label                | `frontend/src/lib/timeline/dateLabel.ts`          | `dateLabel.spec.ts` › `renders a DAY date localized in German`, `renders a DAY date localized in English`                                                                                                                  | Done    |
 | REQ-003 | `UNKNOWN` → `null` (no chip)                                           | #778     | timeline-date-label                | `frontend/src/lib/timeline/dateLabel.ts`          | `dateLabel.spec.ts` › `returns null for UNKNOWN precision even with a date`, `returns null for UNKNOWN precision without a date`                                                                                           | Done    |
@@ -43,11 +43,12 @@
 | REQ-006 | `timeline` in coverage `include` (80% branch gate)                     | #778     | timeline-date-label                | `frontend/vite.config.ts`                         | coverage `include` now lists `src/lib/timeline/**`; covered by all `dateLabel.spec.ts` cases                                                                                                                               | Done    |
 
 <!-- Append real features below this line, one row per REQ-NNN, with the real issue number. Keep the header row above. -->
+
 | REQ-001 | family-member Person with birthDate → 1 BIRTH event, precision passed through | #776 | derive-person-life-events | `timeline/TimelineEventService#buildBirthEvents` | `DerivedEventsAssemblyTest#should_emit_one_geburt_for_person_with_birthdate`, `#should_pass_birth_precision_through_unchanged` | Done |
 | REQ-002 | family-member Person with deathDate → 1 DEATH event | #776 | derive-person-life-events | `timeline/TimelineEventService#buildDeathEvents` | `DerivedEventsAssemblyTest#should_emit_one_tod_for_person_with_deathdate`, `#should_emit_one_tod_and_zero_geburt_for_person_with_deathdate_only` | Done |
 | REQ-003 | null birthDate → 0 Geburt events | #776 | derive-person-life-events | `timeline/TimelineEventService#buildBirthEvents` | `DerivedEventsAssemblyTest#should_emit_zero_tod_when_person_has_birthdate_but_no_deathdate`, `#should_emit_one_tod_and_zero_geburt_for_person_with_deathdate_only` | Done |
 | REQ-004 | null deathDate → 0 Tod events; no dates → 0 events | #776 | derive-person-life-events | `timeline/TimelineEventService#buildDeathEvents` | `DerivedEventsAssemblyTest#should_emit_no_events_for_person_with_neither_date` | Done |
-| REQ-005 | SPOUSE_OF edge with fromYear → MARRIAGE event, eventDate={fromYear}-01-01, precision=YEAR | #776 | derive-person-life-events | `timeline/TimelineEventService#buildMarriageEvents` | `DerivedEventsAssemblyTest#should_emit_one_heirat_for_spouse_edge_with_fromYear` | Done |
+| REQ-005 | SPOUSE*OF edge with fromYear → MARRIAGE event, eventDate={fromYear}-01-01, precision=YEAR | #776 | derive-person-life-events | `timeline/TimelineEventService#buildMarriageEvents` | `DerivedEventsAssemblyTest#should_emit_one_heirat_for_spouse_edge_with_fromYear` | Done |
 | REQ-006 | SPOUSE_OF edge with null fromYear → MARRIAGE emitted, eventDate=null, precision=UNKNOWN | #776 | derive-person-life-events | `timeline/TimelineEventService#buildMarriageEvents` | `DerivedEventsAssemblyTest#should_emit_unknown_precision_heirat_when_fromYear_is_null` | Done |
 | REQ-007 | exactly one MARRIAGE per SPOUSE_OF row (dedup on relationship id) | #776 | derive-person-life-events | `timeline/TimelineEventService#buildMarriageEvents` | `DerivedEventsAssemblyTest#should_emit_exactly_one_heirat_when_both_spouses_in_scope`, `#should_emit_two_heirat_for_person_married_to_two_partners` | Done |
 | REQ-008 | synthetic prefixed ids (birth:/death:/marriage:), never parseable as UUID | #776 | derive-person-life-events | `timeline/TimelineEventService#buildBirthEvents,#buildDeathEvents,#buildMarriageEvents` | `DerivedEventsAssemblyTest#should_mint_prefixed_synthetic_ids_never_uuid` | Done |
@@ -182,7 +183,7 @@
 | REQ-007 | sticky "Filter (N aktiv)" trigger; N from isDefaultState/hiddenLayerCount; 44px target | #780 | timeline-layer-filter | `frontend/src/lib/timeline/timelineFilter.ts`, `frontend/src/lib/timeline/TimelineFilters.svelte` | `timelineFilter.spec.ts#isDefaultState`, `#hiddenLayerCount`; `TimelineFilters.svelte.spec.ts#shows a plain trigger ... and a count`, `#gives the trigger a 44px touch target` | Done |
 | REQ-008 | reset text button restores all layers on; visibility tracks a $derived any-layer-off flag | #780 | timeline-layer-filter | `frontend/src/lib/timeline/TimelineFilters.svelte` | `TimelineFilters.svelte.spec.ts#hides the reset button by default and restores all layers when activated` | Done |
 | REQ-009 | prefers-reduced-motion → slide duration 0 (matchMedia guard reused from documents/[id]/+page.svelte:57) | #780 | timeline-layer-filter | `frontend/src/lib/timeline/TimelineFilters.svelte` | manual reduced-motion check + svelte-autofixer/code review (guard reused, only the slide `duration` zeroed) | Done |
-| REQ-010 | 8 timeline_filter_* keys in de/en/es; trigger vs trigger_active({count}) distinct | #780 | timeline-layer-filter | `frontend/messages/{de,en,es}.json` | `messages.spec.ts#zeitstrahl layer-filter keys are present in all locales (#780 REQ-010)` | Done |
+| REQ-010 | 8 timeline_filter*_ keys in de/en/es; trigger vs trigger*active({count}) distinct | #780 | timeline-layer-filter | `frontend/messages/{de,en,es}.json` | `messages.spec.ts#zeitstrahl layer-filter keys are present in all locales (#780 REQ-010)` | Done |
 | REQ-001 | WRITE_ALL viewer → "Ereignis hinzufügen" link to /zeitstrahl/events/new in the wrapping Zeitstrahl header | #842 | timeline-curator-affordances | `frontend/src/routes/zeitstrahl/+page.svelte`, `frontend/messages/{de,en,es}.json` | `zeitstrahl/page.svelte.spec.ts#renders the add-event CTA in a wrapping header when the viewer can write` | Done |
 | REQ-002 | viewer without WRITE_ALL → no add-event affordance on /zeitstrahl | #842 | timeline-curator-affordances | `frontend/src/routes/zeitstrahl/+page.svelte` | `zeitstrahl/page.svelte.spec.ts#renders no add-event CTA when the viewer cannot write` | Done |
 | REQ-003 | WRITE_ALL viewer → person-page "Ereignis für diese Person" link to /zeitstrahl/events/new?personId={id} | #842 | timeline-curator-affordances | `frontend/src/routes/persons/[id]/PersonCard.svelte`, `frontend/messages/{de,en,es}.json` | `PersonCard.svelte.spec.ts#shows an add-event link pre-seeded with the person to a curator` | Done |
@@ -194,3 +195,23 @@
 | REQ-009 | TimelineView threads canWrite through the year-band path and the undated bucket (identical gate) | #842 | timeline-curator-affordances | `frontend/src/lib/timeline/TimelineView.svelte`, `frontend/src/lib/timeline/YearBand.svelte` | `TimelineView.svelte.spec.ts#threads canWrite to a curated event in both a year band and the undated bucket`, `#threads canWrite to a curated HISTORICAL world band in both paths` | Done |
 | REQ-010 | person add-event opens #781 create form prefilled with the person, returns to /persons/{id} on save | #842 | timeline-curator-affordances | `frontend/src/routes/persons/[id]/PersonCard.svelte`, `frontend/src/routes/zeitstrahl/events/new/+page.server.ts` (#781) | `PersonCard.svelte.spec.ts#shows an add-event link pre-seeded with the person to a curator` (URL), `zeitstrahl/events/new/page.server.spec.ts#redirects to /persons/{id} when originPersonId is a valid UUID` (#781) | Done |
 | REQ-011 | non-curator direct nav to /zeitstrahl/events/new or /{id}/edit → 403 (existing #781 route guard, regression) | #842 | timeline-curator-affordances | `frontend/src/routes/zeitstrahl/events/new/+page.server.ts`, `frontend/src/routes/zeitstrahl/events/[id]/edit/+page.server.ts` (#781, unchanged — both share `requireWriteAll`) | `zeitstrahl/events/new/page.server.spec.ts#throws 403 for an authenticated user without WRITE_ALL`, `zeitstrahl/events/[id]/edit/page.server.spec.ts#throws 403 for a user without WRITE_ALL` | Done |
+| REQ-001 | axis-fixed layers (life-events, pills, world-bands) render identically across all 3 modes; only loose letters re-bundle | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/TimelineView.svelte`, `frontend/src/lib/timeline/YearBand.svelte` | `grouping-event-layer-identity.svelte.spec.ts#renders the event pills and world-bands identically across all three grouping modes`, `YearBand.svelte.spec.ts#still renders the event world-band in Ereignis mode` | Done |
+| REQ-002 | mode switch re-bundles loose letters over the layer-filtered view, no GET /api/timeline refetch | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.svelte`, `frontend/src/lib/timeline/timelineGrouping.ts`, `frontend/src/lib/timeline/TimelineView.svelte` | `zeitstrahl/page.svelte.spec.ts#regroups loose letters under their event client-side`, `e2e/zeitstrahl-grouping.spec.ts#switching grouping modes issues no extra timeline fetch` | Done |
+| REQ-003 | Ereignis clusters each loose letter under the curated event whose documents contain it | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts#bucketLetters`, `frontend/src/lib/timeline/YearBand.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte` | `timelineGrouping.spec.ts#clusters letters under the curated event named by linkedEventId`, `YearBand.svelte.spec.ts#clusters loose letters under their linked event in Ereignis mode` | Done |
+| REQ-004 | Thema buckets each loose letter per year under its primary root tag (rootTagId) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts#bucketLetters`, `frontend/src/lib/timeline/YearBand.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte` | `timelineGrouping.spec.ts#buckets letters under their primary root tag with name and colour`, `YearBand.svelte.spec.ts#buckets loose letters under their root tag in Thema mode` | Done |
+| REQ-005 | TimelineEntryDTO carries nullable linkedEventId, resolved in one batched membership pass | #827 | timeline-grouping-modes | `backend/.../timeline/TimelineEntryDTO.java`, `backend/.../timeline/TimelineService.java#resolveLetterEventLinks`, `frontend/src/lib/generated/api.ts` | `TimelineServiceTest#letter_in_a_curated_events_documents_carries_that_events_id` | Done |
+| REQ-005b | linkedEventId is nullable / not @Schema REQUIRED; null for non-letter entries | #827 | timeline-grouping-modes | `backend/.../timeline/TimelineEntryDTO.java`, `frontend/src/lib/generated/api.ts` (`linkedEventId?`) | `TimelineServiceTest#letter_in_no_curated_event_has_null_linkedEventId` | Done |
+| REQ-006 | Ereignis: letter with null linkedEventId → per-year "Weitere Briefe" bucket | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts`, `frontend/src/lib/timeline/LetterBucket.svelte` | `timelineGrouping.spec.ts#drops a letter with no linkedEventId into the fallback bucket`, `LetterBucket.svelte.spec.ts#uses the localized "Weitere Briefe" label` | Done |
+| REQ-007 | Thema: untagged letter → per-year "Ohne Thema" bucket | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts`, `frontend/src/lib/timeline/LetterBucket.svelte` | `timelineGrouping.spec.ts#drops an untagged letter into the "Ohne Thema" fallback bucket`, `LetterBucket.svelte.spec.ts#uses the localized "Ohne Thema" label` | Done |
+| REQ-008 | multi-tagged letter appears under exactly one root tag, never duplicated | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts` | `timelineGrouping.spec.ts#places a letter in exactly one bucket` | Done |
+| REQ-009 | tag names + hint render via `{...}` escaping; grep gate forbids `{@html}` in lib/timeline | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/BucketHeaderChip.svelte`, `frontend/src/lib/timeline/LetterCard.svelte`, `frontend/src/lib/timeline/TagChip.svelte` | `BucketHeaderChip.svelte.spec.ts#renders an HTML-bearing name as inert text`, `timeline-no-raw-html.spec.ts#no timeline component contains the raw-HTML directive` | Done |
+| REQ-010 | grouping control is a keyboard-navigable role=radiogroup, ≥44px text segments, default Datum, dark-mode contrast | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/GroupingControl.svelte` | `GroupingControl.svelte.spec.ts#renders three radios inside a radiogroup`, `#moves the selection forward with the right arrow key`, `#each segment has a tap target of at least 44×44px`, `#defaults to Datum`; `e2e/zeitstrahl-grouping.spec.ts#no wcag2a/wcag2aa violations ... (light + dark)` | Done |
+| REQ-011 | ≤320px: control overflow-free + tap ≥44px, each abbreviation carries its full word as aria-label | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/GroupingControl.svelte` | `GroupingControl.svelte.spec.ts#exposes each segment full word as an aria-label`, `e2e/zeitstrahl-grouping.spec.ts#the control stays overflow-free and operable at 320px` | Done |
+| REQ-012 | new grouping/bucket Paraglide keys in de/en/es; no collision with existing timeline*_ keys | #827 | timeline-grouping-modes | `frontend/messages/{de,en,es}.json` | `messages.spec.ts#zeitstrahl grouping + bucket keys are present in all locales (#827 REQ-012)`, `messages.spec.ts#de, en, and es have identical key sets` | Done |
+| REQ-013 | failed timeline fetch → existing localized error via getErrorMessage; grouping has no independent failure mode | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.server.ts` (#779, unchanged) | `zeitstrahl/page.server` error path (#779 — getErrorMessage(extractErrorCode)) | Done |
+| REQ-014 | Ereignis event-clustered letter renders as the `.lcard.ev` variant | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterCard.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte` | `LetterCard.svelte.spec.ts#carries the .lcard.ev class in the event variant`, `LetterBucket.svelte.spec.ts#renders its letters as .lcard.ev event cards` | Done |
+| REQ-015 | Thema bucket-header chip tinted via rootTagColor `var(--c-tag-*)`, neutral fallback when null/unknown | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/BucketHeaderChip.svelte` | `BucketHeaderChip.svelte.spec.ts#tints the chip with var(--c-tag-{token})`, `#renders a neutral chip with no --c-tag- binding when colour is null`, `#falls back to neutral for an unknown colour token` | Done |
+| REQ-016 | header meta-line grouping segment tracks the active mode (date/event/thema keys) | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.svelte` | `zeitstrahl/page.svelte.spec.ts#updates the meta-line grouping label when a mode is chosen` | Done |
+| REQ-017 | Thema: per-letter TagChip suppressed inside its own bucket; still shown in Datum/Ereignis | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterCard.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte` | `LetterCard.svelte.spec.ts#suppresses the per-letter tag chip when asked`, `#still shows the per-letter tag chip when not suppressed`, `LetterBucket.svelte.spec.ts#suppresses the per-letter tag chip inside its own root-tag bucket` | Done |
+| REQ-018 | Letters layer off → grouping control disabled (kept in place), mode retained | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.svelte`, `frontend/src/lib/timeline/GroupingControl.svelte`, `frontend/src/lib/timeline/timelineGrouping.ts#hasLooseLetters` | `zeitstrahl/page.svelte.spec.ts#disables the grouping control when the Letters layer is off`, `GroupingControl.svelte.spec.ts#retains the active mode while disabled`, `timelineGrouping.spec.ts#hasLooseLetters` | Done |
+| REQ-019 | Ereignis: letter whose only linking event was filtered off → "Weitere Briefe" (never re-introduced) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts#buildEventLookup`, `frontend/src/lib/timeline/TimelineView.svelte` | `timelineGrouping.spec.ts#drops a letter whose linked event is absent from the lookup into fallback` | Done |

docs/adr/045-timeline-client-side-regroup.md

@@ -0,0 +1,78 @@
+# ADR-045 — The /zeitstrahl Ereignis/Thema regroup is client-side, over a computed letter→event link
+
+**Status:** Accepted
+**Date:** 2026-06-15
+**Issue:** #827 (Zeitstrahl milestone; deferred follow-up to #779, builds on #835/PR #838 and #780)
+
+## Context
+
+#779 shipped `/zeitstrahl` in **Datum** mode only and deferred the Concept-A
+**Datum · Ereignis · Thema** segmented control, because the other two modes need data the
+`TimelineEntryDTO` did not carry: a letter's curated-event association (Ereignis) and a letter's
+primary root tag + colour (Thema). #835 (merged in PR #838) added the Thema fields
+(`rootTagId`/`rootTagName`/`rootTagColor`) and the batched `TimelineService → TagService`
+resolver. Meanwhile #780 added the **layer filter** — `/zeitstrahl/+page.svelte` owns
+`personalOn`/`historicalOn`/`lettersOn` `$state` and renders `TimelineView` over a client-side
+`filterTimeline(data.timeline, …)` view.
+
+This ADR records the three forks specific to **#827** (the Thema enrichment + the
+`TimelineService → TagService` edge are #835's scope, not this one).
+
+## Decisions
+
+### 1. Grouping is a client-side presentation transform — no `grouping=` query param
+
+`GET /api/timeline` already returns the whole timeline in one payload. Regrouping the loose
+letters is an in-memory transform in `lib/timeline/timelineGrouping.ts` (`bucketLetters`,
+`buildEventLookup`, `hasLooseLetters`), driven by a `groupingMode` `$state` in `+page.svelte`.
+A server-side `grouping=DATE|EVENT|TOPIC` parameter was rejected: it would add lasting API
+surface and a bucket query for zero benefit on an already-loaded payload, and switching modes
+must issue **zero** extra fetches (REQ-002). The blast radius stays inside the read view.
+
+### 2. The letter→event link is computed, reusing `timeline_event_documents` — no new column
+
+A letter clusters under a curated event iff that event's `documents` set (ADR-040;
+`@ManyToMany @BatchSize(50)` over join table `timeline_event_documents`) contains the letter's
+document. `TimelineService.assemble` resolves this in **one batched membership pass** —
+`resolveLetterEventLinks` builds a single `docId → eventId` map over the already-loaded events
+(no per-letter query), reusing the same `eventRepository.findAll()` it already iterates for the
+event entries. The result is exposed as one nullable DTO field, `linkedEventId`. A new persisted
+FK on the document/letter row was rejected: it duplicates an existing capability and opens a
+mutating write path + Flyway migration for no gain. **No new column, no migration, no new
+cross-domain edge** (the field derives from data `TimelineService` already loads). `linkedEventId`
+is deliberately **not** `@Schema(requiredMode = REQUIRED)` — it is null for non-letter entries and
+for letters under no curated event — so the generated TypeScript type stays optional.
+
+### 3. Grouping composes with the #780 layer filter as **filter-then-group**
+
+The pipeline is `data.timeline → filterTimeline() (#780) → groupingMode transform → TimelineView`.
+The grouping `$state` lives in `+page.svelte` beside the filter `$state`, and the regroup runs over
+the layer-**filtered** view, never the raw `data.timeline`. Grouping the raw timeline and filtering
+afterward was rejected: the counts and buckets would disagree with the layer toggles, re-opening
+the #780 count-mismatch the page already closed. Two consequences fall out of filter-then-group:
+
+- **Letters layer off → the grouping control disables, kept in place (REQ-018).** With no loose
+  letters in the filtered view there is nothing to regroup; the control renders `aria-disabled`
+  (no header reflow), keeps its selected mode, and announces a screen-reader reason.
+- **A letter whose only linking event was filtered out falls back to "Weitere Briefe" (REQ-019).**
+  `buildEventLookup` is built from the events present in the _filtered_ view, so Ereignis clusters
+  only under events that survived the filter; everything else lands in the per-year fallback bucket.
+
+The control is a `role="radiogroup"` (single-select), deliberately distinct from #780's
+`aria-pressed` toggle filter, stacked above the filter trigger so the two read as one control
+cluster — the top-right corner stays the #842 add-event CTA.
+
+## Consequences
+
+- One nullable field (`linkedEventId`) is added to `TimelineEntryDTO` (17 components); the
+  regenerated `frontend/src/lib/generated/api.ts` is committed in the same PR. No table, column,
+  Flyway migration, endpoint, `ErrorCode`, or `Permission` changes.
+- The regroup is pure and fully unit-tested independently of the components; `TimelineView`/
+  `YearBand` render the axis-fixed event layer identically across all three modes (REQ-001) and
+  only swap the loose-letter rendering for per-year `LetterBucket`s off Datum.
+- The new Thema bucket-header chip (`BucketHeaderChip`) is a filled variant tinted from
+  `rootTagColor`; the shipped neutral per-letter `TagChip` (#838) is reused as-is and suppressed
+  inside its own bucket (REQ-017). All `lib/timeline` components keep the `{...}`-escaping
+  guarantee — a grep gate forbids `{@html}` (REQ-009).
+- Read-only feature: no new authn/authz surface beyond the existing `READ_ALL` on
+  `GET /api/timeline`.

frontend/CLAUDE.md

@@ -50,7 +50,7 @@ src/
 │   │   ├── relationship/ # Relationship form + chip components
 │   │   └── genealogy/   # Stammbaum (family tree) components
 │   ├── tag/             # Tag domain: TagInput, TagChipList, TagParentPicker
-│   ├── timeline/        # Timeline (Zeitstrahl) domain: TimelineView, YearBand, EventPill, WorldBand, LetterCard, YearLetterStrip, GapSpan; dateLabel + timelineDensity + eventCardConfig (imports $lib/shared only, never document/)
+│   ├── timeline/        # Timeline (Zeitstrahl) domain: TimelineView, YearBand, EventPill, WorldBand, LetterCard, LetterBucket, BucketHeaderChip, GroupingControl, TagChip, YearLetterStrip, GapSpan; dateLabel + timelineDensity + timelineFilter + timelineGrouping + eventCardConfig (imports $lib/shared only, never document/)
 │   ├── geschichte/      # Geschichte (story) domain: editor + card
 │   ├── notification/    # Notification bell + dropdown + store
 │   ├── activity/        # Activity feed (Chronik) components