feat: prune orphaned photo rows + retry-pending action

Adds /api/v1/library/maintenance/{missing-stats,prune-missing} backed
by a new cleanup helper that deletes Photo rows whose files no longer
exist on disk under a *mounted* source root. Skips photos under
unmounted roots so a temporarily-disconnected drive doesn't get
silently nuked.

Settings panel surfaces the orphan count with a destructive Prune
button, plus a "Kick pending" action that re-queues photos stuck in
processing_status='pending' (typically left behind when the scanner
created the row but the worker never picked up the thumbnail task).

Common trigger: PHOTO_DIRS in .env was repointed at a different
library root, leaving every old row dangling.

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

frontend/src/services/api.ts

@@ -230,6 +230,18 @@ export interface RegenerateResult {
   }
 }
 
+export interface MissingStats {
+  would_delete?: number
+  deleted?: number
+  skipped_unmounted: number
+  dry_run: boolean
+}
+
+export interface PruneResult extends MissingStats {
+  status?: string
+  message?: string
+}
+
 export interface WorkerInfo {
   name: string
   status: 'online' | 'unreachable'
@@ -295,7 +307,11 @@ export const library = {
     /** 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 } = {}
+      body: {
+        media_types?: MediaType[]
+        only_failed?: boolean
+        only_pending?: boolean
+      } = {}
     ): Promise<RegenerateResult> => {
       const response = await api.post(
         '/library/maintenance/regenerate-thumbnails',
@@ -312,6 +328,19 @@ export const library = {
       return response.data
     },
 
+    /** Dry-run count of photo rows whose files are no longer on disk
+     *  (under a mounted source root). */
+    missingStats: async (): Promise<MissingStats> => {
+      const response = await api.get('/library/maintenance/missing-stats')
+      return response.data
+    },
+
+    /** Actually delete the orphaned photo rows. */
+    pruneMissing: async (): Promise<PruneResult> => {
+      const response = await api.post('/library/maintenance/prune-missing')
+      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 }> => {