feat(backend): add AdminController endpoints for thumbnail backfill

- POST /api/admin/generate-thumbnails  → triggers async backfill, 202
- GET  /api/admin/thumbnail-status     → returns current BackfillStatus

Both gated by the class-level @RequirePermission(Permission.ADMIN).
Shape and polling semantics mirror the mass-import endpoints.

Refs #307

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

backend/src/main/java/org/raddatz/familienarchiv/controller/AdminController.java

@@ -6,6 +6,7 @@ import org.raddatz.familienarchiv.security.RequirePermission;
 import org.raddatz.familienarchiv.service.DocumentService;
 import org.raddatz.familienarchiv.service.DocumentVersionService;
 import org.raddatz.familienarchiv.service.MassImportService;
+import org.raddatz.familienarchiv.service.ThumbnailBackfillService;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PostMapping;
@@ -23,6 +24,7 @@ public class AdminController {
     private final MassImportService massImportService;
     private final DocumentService documentService;
     private final DocumentVersionService documentVersionService;
+    private final ThumbnailBackfillService thumbnailBackfillService;
 
     @PostMapping("/trigger-import")
     public ResponseEntity<MassImportService.ImportStatus> triggerMassImport() {
@@ -47,4 +49,15 @@ public class AdminController {
         int count = documentService.backfillFileHashes();
         return ResponseEntity.ok(new BackfillResult(count));
     }
+
+    @PostMapping("/generate-thumbnails")
+    public ResponseEntity<ThumbnailBackfillService.BackfillStatus> generateThumbnails() {
+        thumbnailBackfillService.runBackfillAsync();
+        return ResponseEntity.accepted().body(thumbnailBackfillService.getStatus());
+    }
+
+    @GetMapping("/thumbnail-status")
+    public ResponseEntity<ThumbnailBackfillService.BackfillStatus> thumbnailStatus() {
+        return ResponseEntity.ok(thumbnailBackfillService.getStatus());
+    }
 }