feat(timeline): add ErrorCodes for event CRUD

Add TIMELINE_EVENT_NOT_FOUND (404), TIMELINE_EVENT_CONFLICT (409),
TIMELINE_TITLE_TOO_LONG (400), and a generic CONFLICT (409) used by the
optimistic-lock backstop. Per #775 / ADR-040.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -155,6 +155,14 @@ public enum ErrorCode {
     /** The merge target is a descendant of the source tag. 400 */
     TAG_MERGE_INVALID_TARGET,
 
+    // --- Timeline (Zeitstrahl) ---
+    /** A timeline event with the given ID does not exist. 404 */
+    TIMELINE_EVENT_NOT_FOUND,
+    /** Optimistic-locking conflict — the timeline event was modified by another curator. 409 */
+    TIMELINE_EVENT_CONFLICT,
+    /** A timeline event title exceeds the maximum length (255 characters — the DB column bound). 400 */
+    TIMELINE_TITLE_TOO_LONG,
+
     // --- Generic ---
     /** Request validation failed (missing or malformed fields). 400 */
     VALIDATION_ERROR,
@@ -162,6 +170,8 @@ public enum ErrorCode {
     BATCH_TOO_LARGE,
     /** Bulk edit request exceeds the per-request document ID cap. 400 */
     BULK_EDIT_TOO_MANY_IDS,
+    /** A concurrent modification was detected (generic optimistic-lock backstop). 409 */
+    CONFLICT,
     /** An unexpected server-side error occurred. 500 */
     INTERNAL_ERROR,
 }