feat(maintenance): re-extract archive attachments + link to post — #713 part 2

Existing PostAttachments that are actually archives (filed opaquely before the
magic-byte gate) need extracting retroactively. cleanup_service.
reextract_archive_attachments scans PostAttachments, magic-detects the archives,
and for each reconstructs the post's sidecar from the DB + re-runs attach_in_place
in a temp dir — so the members extract and re-link to the SAME post via
find_or_create_post (source_id + external_post_id). Idempotent (members dedupe by
sha256). Enqueues thumbnail+ML for new members.

Wired as a maintenance-queue Celery task (tasks/admin) + POST
/api/admin/maintenance/reextract-archives (202) + a "Re-extract archive
attachments" card in Settings → Maintenance.

Test: a zip stored under a mangled extension-less name extracts + links its
member to the post via ImageProvenance, and a second run is a no-op (idempotent).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/tasks/admin.py

@@ -55,3 +55,21 @@ def bulk_delete_images_task(self, *, image_ids: list[int]) -> dict:
         return cleanup_service.delete_images(
             session, image_ids=image_ids, images_root=IMAGES_ROOT,
         )
+
+
+@celery.task(
+    name="backend.app.tasks.admin.reextract_archive_attachments_task",
+    bind=True,
+    autoretry_for=(OperationalError, DBAPIError),
+    retry_backoff=15, retry_backoff_max=180, max_retries=1,
+    soft_time_limit=1800, time_limit=2400,  # 30 min / 40 min
+)
+def reextract_archive_attachments_task(self) -> dict:
+    """Wraps cleanup_service.reextract_archive_attachments (#713 part 2):
+    re-extract PostAttachments that are actually archives but were filed
+    opaquely before the magic-byte gate, and link their members to the post."""
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        return cleanup_service.reextract_archive_attachments(
+            session, images_root=IMAGES_ROOT,
+        )