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/serializers.py

@@ -4,7 +4,7 @@ from pathlib import Path
 from django.core import signing
 from rest_framework import serializers
 
-from .models import MediaItem, MediaLocation, TempUpload, DownloadTask
+from .models import MediaItem, MediaLocation, TempUpload, DownloadTask, MatchTask
 from .services import (
     MEDIA_FILE_SALT,
     UPLOAD_FILE_SALT,
@@ -55,6 +55,8 @@ class MediaItemSerializer(serializers.ModelSerializer):
             "uploaded_by_id",
             "e621_post_id",
             "e621_data",
+            "e621_match_status",
+            "e621_checked_at",
             "locations",
             "created_at",
             "updated_at",
@@ -74,6 +76,8 @@ class MediaItemSerializer(serializers.ModelSerializer):
             "uploaded_by_id",
             "e621_post_id",
             "e621_data",
+            "e621_match_status",
+            "e621_checked_at",
             "locations",
             "created_at",
             "updated_at",
@@ -220,3 +224,25 @@ class DownloadTaskSerializer(serializers.ModelSerializer):
 
     def get_library_j_id(self, obj):
         return f"J-{obj.library_item_id}" if obj.library_item_id else None
+
+
+class MatchTaskSerializer(serializers.ModelSerializer):
+    task_id = serializers.UUIDField(source="id", read_only=True)
+
+    class Meta:
+        model = MatchTask
+        fields = [
+            "task_id",
+            "scope",
+            "status",
+            "total",
+            "processed",
+            "matched",
+            "not_found",
+            "deleted",
+            "failed",
+            "error",
+            "created_at",
+            "updated_at",
+        ]
+        read_only_fields = fields