feat: worker diagnostics in settings panel

Adds /api/v1/library/maintenance/worker-status (Celery inspect + queue
depths + recent failed photos) and a Workers section in the Settings
dialog so users can debug stuck queues and task failures without
tailing container logs. Auto-polls every 5s while open.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -230,6 +230,45 @@ export interface RegenerateResult {
   }
 }
 
+export interface WorkerInfo {
+  name: string
+  status: 'online' | 'unreachable'
+  active: number
+  reserved: number
+  scheduled: number
+  concurrency: number | null
+  processed: Record<string, number>
+  queues: string[]
+  active_tasks: Array<{
+    id: string
+    name: string
+    args: unknown
+    time_start: number | null
+  }>
+}
+
+export interface WorkerFailure {
+  photo_id: string
+  filename: string
+  media_type: string
+  error: string
+  updated_at: string | null
+}
+
+export interface WorkerStatus {
+  broker_ok: boolean
+  broker_error: string | null
+  inspect_error: string | null
+  workers: WorkerInfo[]
+  worker_count: number
+  queues: Record<string, number>
+  failures: {
+    total: number
+    recent: WorkerFailure[]
+  }
+  scan_errors: string[]
+}
+
 export const library = {
   scan: async () => {
     const response = await api.post('/library/scan')
@@ -265,6 +304,14 @@ export const library = {
       return response.data
     },
 
+    /** Celery worker fleet diagnostics + recent task failures. Surfaced
+     *  in the Settings panel so users can debug stuck queues without
+     *  tailing container logs. */
+    workerStatus: async (): Promise<WorkerStatus> => {
+      const response = await api.get('/library/maintenance/worker-status')
+      return response.data
+    },
+
     /** Re-run the source-roots / folders / photos integrity cleanup that
      *  normally runs on backend startup. */
     cleanup: async (): Promise<{ status: string; message?: string }> => {