feat(backend): add THUMBNAIL_BACKFILL_ALREADY_RUNNING error code

Mirrors the IMPORT_ALREADY_RUNNING pattern for the concurrent-start
guard in ThumbnailBackfillService.

Refs #307

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

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

@@ -38,6 +38,10 @@ public enum ErrorCode {
     /** A mass import is already in progress; only one can run at a time. 409 */
     IMPORT_ALREADY_RUNNING,
 
+    // --- Thumbnails ---
+    /** A thumbnail backfill is already in progress; only one can run at a time. 409 */
+    THUMBNAIL_BACKFILL_ALREADY_RUNNING,
+
     // --- Invites ---
     /** The invite code does not exist. 404 */
     INVITE_NOT_FOUND,