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/models.py

@@ -27,6 +27,17 @@ def build_search_tags(tags, e621_data):
 class MediaItem(models.Model):
     """A logical media file, identified by its MD5 fingerprint."""
 
+    E621_UNKNOWN = "unknown"
+    E621_MATCHED = "matched"
+    E621_NOT_FOUND = "not_found"
+    E621_DELETED = "deleted"
+    E621_STATUS_CHOICES = [
+        (E621_UNKNOWN, "Not checked"),
+        (E621_MATCHED, "Matched"),
+        (E621_NOT_FOUND, "Not found on e621"),
+        (E621_DELETED, "Post deleted on e621"),
+    ]
+
     md5 = models.CharField(max_length=32, unique=True)
     size = models.BigIntegerField(default=0)
     # Custom rating: "s" (safe), "q" (questionable), "e" (explicit) or "" (unrated).
@@ -44,6 +55,14 @@ class MediaItem(models.Model):
     hidden_from_guests = models.BooleanField(default=False, db_index=True)
     e621_post_id = models.IntegerField(null=True, blank=True, db_index=True)
     e621_data = models.JSONField(null=True, blank=True)
+    # Cached result of the last MD5/manual match against e621.
+    e621_match_status = models.CharField(
+        max_length=20,
+        choices=E621_STATUS_CHOICES,
+        default=E621_UNKNOWN,
+        db_index=True,
+    )
+    e621_checked_at = models.DateTimeField(null=True, blank=True)
     # Perceptual hashes (hex strings) used by the duplicates engine.
     ahash = models.CharField(max_length=32, blank=True, default="", db_index=True)
     dhash = models.CharField(max_length=32, blank=True, default="", db_index=True)
@@ -207,3 +226,58 @@ class DownloadTask(models.Model):
 
     def __str__(self):
         return f"Download {self.id} ({self.status})"
+
+
+class MatchTask(models.Model):
+    """A background library-wide e621 match scan with progress tracking."""
+
+    STATUS_PENDING = "pending"
+    STATUS_RUNNING = "running"
+    STATUS_COMPLETE = "complete"
+    STATUS_ERROR = "error"
+    STATUS_CANCELLED = "cancelled"
+    STATUS_CHOICES = [
+        (STATUS_PENDING, "Pending"),
+        (STATUS_RUNNING, "Running"),
+        (STATUS_COMPLETE, "Complete"),
+        (STATUS_ERROR, "Error"),
+        (STATUS_CANCELLED, "Cancelled"),
+    ]
+
+    SCOPE_MISSING = "missing"
+    SCOPE_ALL = "all"
+    SCOPE_CHOICES = [
+        (SCOPE_MISSING, "Unchecked items"),
+        (SCOPE_ALL, "Whole library"),
+    ]
+
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name="match_tasks",
+    )
+    scope = models.CharField(
+        max_length=20, choices=SCOPE_CHOICES, default=SCOPE_MISSING
+    )
+    status = models.CharField(
+        max_length=20, choices=STATUS_CHOICES, default=STATUS_PENDING
+    )
+    total = models.IntegerField(default=0)
+    processed = models.IntegerField(default=0)
+    matched = models.IntegerField(default=0)
+    not_found = models.IntegerField(default=0)
+    deleted = models.IntegerField(default=0)
+    failed = models.IntegerField(default=0)
+    error = models.TextField(blank=True, default="")
+    cancelled = models.BooleanField(default=False)
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ["-created_at"]
+
+    def __str__(self):
+        return f"Match scan {self.id} ({self.status})"