Match local files to e621: MD5 lookups, manual links, batch scans

- MediaItem gains e621_match_status (unknown/matched/not_found/deleted)
  and e621_checked_at, backfilled for existing matched items.
- Server-side e621 client (apps/library/e621.py) using the user's stored
  credentials, throttled to 2 req/s, with typed errors.
- Matching service: MD5 lookup, manual post linking (flags MD5
  mismatches), unlink, metadata refresh, deleted-post detection.
- Detail actions POST /api/files/J-x/match/ and /unlink/ (uploader or
  staff only).
- Background library scans: MatchTask + /api/matches/ with missing/all
  scopes, progress polling, cancel and stale-task reaping; the scan
  counts toward the footer's Active Workers. Same pass available as
  manage.py match_e621 for cron.
- Library gains not_found/deleted status filters; the detail page adds
  an e621 match card (check / link by post ID / unlink) and the metadata
  card warns when a post was deleted on e621.

backend/apps/library/urls.py

@@ -11,12 +11,18 @@ from .tools import (
     VisualMatchesView,
 )
 from .uploads import TempUploadViewSet
-from .views import ClientDownloadView, DownloadTaskViewSet, MediaItemViewSet
+from .views import (
+    ClientDownloadView,
+    DownloadTaskViewSet,
+    MatchTaskViewSet,
+    MediaItemViewSet,
+)
 
 router = DefaultRouter()
 router.register("files", MediaItemViewSet, basename="file")
 router.register("uploads", TempUploadViewSet, basename="upload")
 router.register("online/downloads", DownloadTaskViewSet, basename="download")
+router.register("matches", MatchTaskViewSet, basename="match")
 
 urlpatterns = [
     path("", include(router.urls)),