feat(person): ErrorCode.PERSON_RENAME_CONFLICT for optimistic-lock conflicts

Adds the structured error code returned when a rename rolls back because a
referenced transcription block was edited concurrently (OptimisticLockException
on transcription_blocks.version). Mirrors the contract in
frontend src/lib/errors.ts and adds the localised message keys
error_person_rename_conflict in de/en/es so the UI surfaces a retry hint
instead of a generic 500.

The actual translation of OptimisticLockException → DomainException
(PERSON_RENAME_CONFLICT) lands in the next commit alongside the integration
test that proves the rollback semantics.

Refs #362

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -15,6 +15,10 @@ public enum ErrorCode {
     ALIAS_NOT_FOUND,
     /** The submitted personType value is not allowed (e.g. SKIP is import-only). 400 */
     INVALID_PERSON_TYPE,
+    /** A concurrent edit on a referenced transcription block prevented the rename
+     * from committing (optimistic-lock conflict). The whole rename rolls back; the
+     * client should refetch and retry. 409 */
+    PERSON_RENAME_CONFLICT,
 
     // --- Documents ---
     /** A document with the given ID does not exist. 404 */