feat(geschichte): JourneyItem CRUD API — append, updateNote, delete, reorder (#751) (#788)

## Summary

Implements the backend JourneyItem CRUD API on top of the data model from #750, building towards the full Lesereisen feature (#751).

**Completed in this PR:**
- `jackson-databind-nullable` 0.2.6 + `JacksonConfig` (`@Bean Module`) for three-way PATCH semantics (`JsonNullable`)
- `AuditKind`: `JOURNEY_ITEM_ADDED`, `JOURNEY_ITEM_REMOVED`, `JOURNEY_ITEMS_REORDERED` (last is rollup-eligible)
- `ErrorCode`: `JOURNEY_ITEM_NOT_FOUND`, `JOURNEY_ITEM_POSITION_CONFLICT`
- V73 migration: `UNIQUE (geschichte_id, position) DEFERRABLE INITIALLY DEFERRED` + `CHECK (position > 0)` on `journey_items`
- `JourneyItemConstraintsTest`: verifies deferrable flag via `pg_constraint` query; position check; duplicate position rejection (3 passing tests)
- Read models: `DocumentSummary`, `JourneyItemView`, `GeschichteView` (with `AuthorView` to prevent AppUser email leak)
- `DocumentService.getSummaryById` — lean lookup without tag-color resolution
- `JourneyItemRepository`: extended with `findByGeschichteIdOrderByPosition`, `findByIdAndGeschichteId` (IDOR-safe), `findIdsByGeschichteId`, `findMaxPositionByGeschichteId`, `countByGeschichteId`
- DTOs: `JourneyItemCreateDTO`, `JourneyItemUpdateDTO` (`JsonNullable<String> note`), `JourneyReorderDTO`

**Still in progress (WIP):**
- `JourneyItemService` — `append`, `updateNote`, `delete`, `reorder`, `toSummary`, `toView` (Task 6)
- `GeschichteService.getById` → returns `GeschichteView` (Task 7)
- New endpoints on `GeschichteController` + slice tests (Task 8)
- Frontend error codes + i18n + `npm run generate:api` (Task 9)

## Commits

- `0b177247` feat(config): add jackson-databind-nullable for JsonNullable PATCH DTO support
- `408ae334` feat(audit,error): add JourneyItem AuditKind values and ErrorCodes
- `7b06c3ad` feat(migration): V73 adds UNIQUE DEFERRABLE and CHECK position > 0 on journey_items
- `160ca1c3` feat(geschichte): add DocumentSummary, JourneyItemView, GeschichteView read models
- `2ad5c36e` feat(geschichte): extend JourneyItemRepository and add item DTOs

## Test plan

- [ ] `./mvnw test -Dtest=JourneyItemConstraintsTest` — all 3 constraint tests pass
- [ ] `./mvnw clean package -DskipTests` — builds clean after remaining tasks are merged
- [ ] Frontend: `npm run generate:api` after Task 9 endpoint additions

Co-authored-by: Marcel <marcel@familienarchiv>
Reviewed-on: #788

docs/GLOSSARY.md

@@ -151,7 +151,13 @@ _See also [Chronik](#chronik-internal)._
 
 **Geschichte** (`Geschichte`) `[user-facing]` — a narrative story or curated document journey published in the archive. Two subtypes: `STORY` (free-form prose linking `Person`s) and `JOURNEY` (a *Lesereise* — an ordered sequence of `JourneyItem`s). Lifecycle: `DRAFT → PUBLISHED` (see `GeschichteStatus`). DRAFT stories are hidden from users without the `BLOG_WRITE` permission.
 
-**JourneyItem** (`JourneyItem`, table `journey_items`) `[internal]` — a single stop in a *Lesereise* (`Geschichte` with `type=JOURNEY`). Either document-backed (`document_id IS NOT NULL`) or a note-only interlude (`note IS NOT NULL`). Ordered by `position` (gaps of 1000 leave room for drag-reorder). A CHECK constraint ensures at least one of `document_id` or `note` is present. The FK to `documents` uses `ON DELETE SET NULL`, so deleting a document preserves the item (with `document_id = null`).
+**JourneyItem** (`JourneyItem`, table `journey_items`) `[internal]` — a single stop in a *Lesereise* (`Geschichte` with `type=JOURNEY`). Either document-backed (`document_id IS NOT NULL`) or a note-only interlude (`note IS NOT NULL`). Ordered by `position` (step of 10; max 100 items per journey). A DEFERRABLE UNIQUE constraint on `(geschichte_id, position)` allows atomic position swaps in the same transaction. A CHECK constraint ensures at least one of `document_id` or `note` is present. The FK to `documents` uses `ON DELETE SET NULL`, so deleting a document preserves the item (with `document_id = null`).
+
+**GeschichteView** (`GeschichteView`) `[internal]` — lean read-model record returned by `GeschichteService.getById()`. Contains `AuthorView` (id + displayName only — email not exposed) and a `List<JourneyItemView>` loaded via a separate query rather than a lazy collection.
+
+**JourneyItemView** (`JourneyItemView`) `[internal]` — lean view record for a single `JourneyItem` surface, containing `id`, `position`, an optional `DocumentSummary`, and an optional `note`.
+
+**DocumentSummary** (`DocumentSummary`) `[internal]` — lean document read-model used inside `JourneyItemView`. Contains title, date, senderName, receiverName, receiverCount, datePrecision — no tags or file storage info.
 
 **Lesereise** `[user-facing]` — a curated reading journey through a sequence of family documents, optionally annotated with editorial notes. Implemented as a `Geschichte` with `type=JOURNEY`. The reader UI (follow-on issue) renders items as a sequential reading experience.