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/architecture/c4/l3-backend-3g-supporting.puml

@@ -18,6 +18,8 @@ System_Boundary(backend, "API Backend (Spring Boot)") {
     Component(sseRegistry, "SseEmitterRegistry", "Spring Component", "In-memory ConcurrentHashMap of Spring SseEmitter instances per user. Handles registration, deregistration, and JSON event broadcasts.")
     Component(geschCtrl, "GeschichteController", "Spring MVC — /api/geschichten", "CRUD for publishable stories (STORY) and reading journeys (JOURNEY). Returns GeschichteSummary projections for list; full Geschichte with JourneyItems for detail. Requires BLOG_WRITE permission for write operations.")
     Component(geschSvc, "GeschichteService", "Spring Service", "Manages story lifecycle (DRAFT → PUBLISHED with timestamp). Supports two subtypes: STORY (prose) and JOURNEY (ordered JourneyItem sequence). Sanitizes HTML body with an allowlist policy.")
+    Component(geschQuerySvc, "GeschichteQueryService", "Spring Service", "Read-only facade over GeschichteRepository. Exposes existsById() and findById() to prevent JourneyItemService from crossing domain boundaries.")
+    Component(journeyItemSvc, "JourneyItemService", "Spring Service", "Manages journey item lifecycle: append (100-item cap), updateNote (three-way PATCH), delete, and reorder (DEFERRABLE position swap). Enforces JOURNEY-type guard on append.")
     Component(exHandler, "GlobalExceptionHandler", "Spring @RestControllerAdvice", "Converts DomainException, validation errors, and generic exceptions to ErrorResponse JSON with machine-readable ErrorCode and HTTP status.")
 }
 
@@ -38,6 +40,10 @@ Rel(notifCtrl, notifSvc, "Delegates to")
 Rel(notifCtrl, sseRegistry, "Registers client SSE connection")
 Rel(notifSvc, sseRegistry, "Broadcasts events to connected clients")
 Rel(geschCtrl, geschSvc, "Delegates to")
+Rel(geschCtrl, journeyItemSvc, "Delegates journey item CRUD")
+Rel(journeyItemSvc, geschQuerySvc, "Checks Geschichte existence and type")
+Rel(geschQuerySvc, db, "Reads geschichten", "JDBC")
+Rel(journeyItemSvc, db, "Reads / writes journey_items", "JDBC")
 Rel(auditSvc, db, "Writes audit_log", "JDBC")
 Rel(auditQuery, db, "Reads audit_log", "JDBC")
 Rel(notifSvc, db, "Reads / writes notifications", "JDBC")