feat(exception): add optimistic-lock backstop returning generic 409

Centralized @ExceptionHandler(ObjectOptimisticLockingFailureException) net so
any write path losing a @Version race becomes a generic 409 (CONFLICT code) —
never a 500 + Sentry + Hibernate internals (CWE-209). No Sentry, class-name-
only parameterized logging, body free of id/version/class. Entity-agnostic by
design (no switch on getPersistentClassName); the service catch keeps the
precise TIMELINE_EVENT_CONFLICT. Per #775 Q2/R4/R8.

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

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

@@ -104,6 +104,30 @@ public class GlobalExceptionHandler {
         return "unknown";
     }
 
+    /**
+     * Generic backstop for optimistic-locking conflicts that escape a service-level catch. A
+     * conflict is a 409, not a system fault — so, like {@link #handleDataIntegrityViolation}, it
+     * must NOT fire Sentry and must NOT leak Hibernate internals (CWE-209): the response carries
+     * only the generic {@link ErrorCode#CONFLICT} code and a generic message — no entity id, no
+     * version, no persistent-class name.
+     *
+     * <p>Deliberately code-GENERIC: do NOT {@code switch} on {@code getPersistentClassName()} to map
+     * back to a per-entity code. Unlike {@link #handleDataIntegrityViolation}, which branches on
+     * stable schema constraint NAMES, persistent-class names are not a contract. The precise,
+     * code-carrying path is the service catch (e.g. {@code TIMELINE_EVENT_CONFLICT}); this is only
+     * the net that keeps any current or future write path from regressing to a 500.
+     */
+    @ExceptionHandler(org.springframework.orm.ObjectOptimisticLockingFailureException.class)
+    public ResponseEntity<ErrorResponse> handleOptimisticLock(
+            org.springframework.orm.ObjectOptimisticLockingFailureException ex) {
+        // Log the persistent-class name ONLY (schema metadata, safe for Loki). Never `ex` /
+        // ex.getMessage(): those embed the entity id + version (CWE-209). No Sentry: it's a 409.
+        log.warn("Rejected a write that lost an optimistic-lock race on: {}", ex.getPersistentClassName());
+        return ResponseEntity.status(409)
+                .body(new ErrorResponse(ErrorCode.CONFLICT,
+                        "The resource was modified concurrently. Please reload and try again."));
+    }
+
     @ExceptionHandler(Exception.class)
     public ResponseEntity<ErrorResponse> handleGeneric(Exception ex) {
         Sentry.captureException(ex);