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/downloads.py

@@ -111,7 +111,14 @@ def run_download_task(task_id):
         if task.e621_data:
             item.e621_post_id = task.post_id
             item.e621_data = task.e621_data
-            update_fields += ["e621_post_id", "e621_data"]
+            item.e621_match_status = MediaItem.E621_MATCHED
+            item.e621_checked_at = timezone.now()
+            update_fields += [
+                "e621_post_id",
+                "e621_data",
+                "e621_match_status",
+                "e621_checked_at",
+            ]
             rating = (task.e621_data or {}).get("rating")
             if not item.rating and rating in {"s", "q", "e"}:
                 item.rating = rating