feat(relationship): editable relationships with LocalDate+DatePrecision dates and notes (#841)

Closes #837

Makes `PersonRelationship` fully editable (type, related person, dates, notes), migrates its dates from `Integer fromYear/toYear` to `LocalDate + DatePrecision` (mirroring the #773 person pattern, ADR-039 / V76), activates the previously-dead `notes` column, and gives the Zeitstrahl's derived **Heirat** events full date precision for free.

Both Open Decisions confirmed as adopted: **no `@Version`** (last-write-wins, single-writer archive) and **`DELETE` ownership-mismatch aligned 403 → 404** (anti-enumeration, matching the new `PUT`).

## What's in it
- **V78** migrates `person_relationships.from_year/to_year` → `from_date`/`to_date` + NOT-NULL `*_date_precision` (default `UNKNOWN`); pre-check abort on corrupt years, `YYYY-01-01`/`YEAR` backfill, 5 named CHECK constraints, year columns dropped.
- **`PUT /api/persons/{id}/relationships/{relId}`** (`@RequirePermission(WRITE_ALL)`) re-runs every create invariant (self / coherence / order / reverse-PARENT_OF / duplicate) and re-flags family membership; orientation preserved per viewpoint.
- New `ErrorCode.INVALID_RELATIONSHIP_DATES` registered in all four sites (§3.6).
- `TimelineEventService` sources the derived marriage date from `SPOUSE_OF.fromDate` + precision.
- Frontend: `RelationshipDateField` (DAY/MONTH/YEAR), upsert-capable `AddRelationshipForm` (pre-fill + notes + in-flight submit lock), `RelationshipChip` Edit affordance, `updateRelationship` server action, read-view date range + notes, `formatRelationshipDateRange` helper. `api.ts` regenerated.
- Docs: ADR-044, db-orm/db-relationships diagrams, DEPLOYMENT §5 deploy note, RTM REQ-001…REQ-019.

## Requirements
All 19 EARS requirements implemented red/green and marked `Done` in `.specify/rtm.md`.

## Test plan
- **Backend** (targeted, green): `RelationshipMigrationTest` (Testcontainers pg16, 8), `RelationshipServiceTest` (22), `RelationshipControllerTest` (15), `RelationshipServiceIntegrationTest` (real DB, 10), `DerivedEventsAssemblyTest` (17), `ArchitectureTest` (14); `clean package` builds.
- **Frontend** (green): `relationshipDates.spec.ts`, `AddRelationshipForm.svelte.spec.ts`, `RelationshipChip.svelte.spec.ts`, `PersonRelationshipsCard.svelte.test.ts`, `page.server.spec.ts`, `messages.spec.ts`. `npm run check` = 798 (below the ~834 baseline); `npm run lint` clean.

## Notes for reviewers
- **Spec deviation:** the edit form was built by making `AddRelationshipForm` upsert-capable rather than a duplicate `EditRelationshipForm` (DRY); RTM rows reference `AddRelationshipForm.svelte.spec.ts`.
- `api.ts` regenerated from the live spec; only relationship-relevant hunks remain (one springdoc `PageableObject` field-reorder pruned).
- **Deploy:** V78 is one-way and not rolling-deploy-safe — stop old JAR → start new JAR (Flyway runs first); targeted `pg_restore -t person_relationships` for rollback. No maintenance window.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Marcel <marcel@familienarchiv>
Reviewed-on: #841

frontend/messages/de.json

@@ -651,6 +651,7 @@
 	"error_invalid_date_range": "Das Enddatum darf nicht vor dem Startdatum liegen.",
 	"error_birth_after_death": "Geburtsdatum muss vor dem Sterbedatum liegen. Tipp: Falls nur das Todesjahr bekannt ist und der Geburtstag spät im selben Jahr lag, bitte das Folgejahr eintragen.",
 	"error_invalid_date_precision": "Datum und Genauigkeit stimmen nicht überein.",
+	"error_invalid_relationship_dates": "Das Ende-Datum darf nicht vor dem Beginn-Datum liegen.",
 	"validation_last_name_required": "Nachname ist Pflichtfeld.",
 	"validation_first_name_required": "Vorname ist Pflichtfeld.",
 	"error_ocr_service_unavailable": "Der OCR-Dienst ist nicht verfügbar.",
@@ -1221,6 +1222,16 @@
 	"relation_form_field_from_year": "Von Jahr",
 	"relation_form_field_to_year": "Bis Jahr",
 	"relation_form_year_placeholder": "z.B. 1920",
+	"relation_label_from_date": "Beginn (Datum)",
+	"relation_label_to_date": "Ende (Datum)",
+	"relation_label_date_precision": "Genauigkeit",
+	"relation_precision_day": "Genaues Datum (Tag)",
+	"relation_precision_month": "Monat bekannt",
+	"relation_precision_year": "Nur Jahreszahl",
+	"relation_label_notes": "Notizen",
+	"relation_notes_placeholder": "Optionaler Hinweis zu dieser Beziehung",
+	"relation_date_placeholder_hint": "Leer lassen, wenn unbekannt",
+	"relation_edit": "Beziehung bearbeiten",
 	"person_relationships_heading": "Beziehungen",
 	"person_relationships_empty": "Noch keine Beziehungen bekannt.",
 	"timeline_aria_label": "Zeitachse Dokumentdichte",