feat: settings panel + thumbnail pipeline fixes

- photos.py: stop crashing in FileResponse when a thumb hasn't been
  generated; return a clean 404 with Retry-After so the frontend can
  back off.
- thumbs.py: fix process_video_thumbnail (overwrite_output, robust
  duration probe across stream/format, eager frame load + temp cleanup)
  so videos stop ending up as the gray placeholder.
- library.py: new /maintenance/* endpoints — thumbnail-stats,
  regenerate-thumbnails (with media_type / only_failed filters), and a
  manual data-integrity cleanup trigger.
- Frontend Settings panel (gear in TopBar) surfacing those endpoints
  plus a re-scan button and live thumbnail status counts.
- PhotoThumbnail: stretch the auto-retry schedule for slow RAW jobs.

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

frontend/src/services/api.ts

@@ -207,6 +207,29 @@ export const photos = {
 }
 
 // Library API
+export type MediaType = 'photo' | 'raw' | 'heic' | 'video'
+
+export interface ThumbnailStats {
+  total: number
+  pending: number
+  processing: number
+  completed: number
+  failed: number
+  by_media_type: Record<string, number>
+}
+
+export interface RegenerateResult {
+  status: string
+  matched: number
+  queued: number
+  cleared_dirs: number
+  file_errors: number
+  filters: {
+    media_types: MediaType[] | null
+    only_failed: boolean
+  }
+}
+
 export const library = {
   scan: async () => {
     const response = await api.post('/library/scan')
@@ -222,6 +245,33 @@ export const library = {
     const response = await api.get('/library/stats')
     return response.data
   },
+
+  /** Maintenance / admin actions surfaced via the Settings panel. */
+  maintenance: {
+    thumbnailStats: async (): Promise<ThumbnailStats> => {
+      const response = await api.get('/library/maintenance/thumbnail-stats')
+      return response.data
+    },
+
+    /** Reset on-disk thumbs and re-queue Celery generation. With no
+     *  filters, every photo in the library is re-queued. */
+    regenerateThumbnails: async (
+      body: { media_types?: MediaType[]; only_failed?: boolean } = {}
+    ): Promise<RegenerateResult> => {
+      const response = await api.post(
+        '/library/maintenance/regenerate-thumbnails',
+        body
+      )
+      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 }> => {
+      const response = await api.post('/library/maintenance/cleanup')
+      return response.data
+    },
+  },
 }
 
 export interface LibraryStats {