feat(cleanup): purge misgrabbed gated-post blurred previews (#874 follow-up)

A one-shot Maintenance action to remove the blurred locked-preview images
the ingester downloaded from tier-gated Patreon posts before #874.

current_user_can_view was never persisted, so the cleanup re-walks each
enabled Patreon source (read-only) to re-derive which posts are gated now
and the blurred filehashes Patreon serves for them, then matches by
CONTENT HASH against stored source_filehash. Because the hash is
content-addressed, a real file downloaded when access existed has a
different hash and can never match — regained-then-lost-access content is
provably spared (operator's hard requirement). NULL source_filehash =>
unverifiable, kept + reported.

On apply: delete matched ImageRecords + files (provenance cascades),
clear seen/dead-letter ledger rows for those hashes so the real media
re-ingests if access returns, and delete gated posts left bare. Shares
one match predicate between preview and apply (rule 93).

- cleanup_service: collect_gated_previews + purge_gated_previews
- tasks.admin: purge_gated_previews_task (async re-walk bridge, timeboxed)
- api.admin: POST /maintenance/purge-gated-previews
- GatedPurgeCard.vue in Settings > Maintenance (preview -> confirm -> apply)
- tests: collect predicate, hash-match delete/spare/unverifiable, ledger
  clear, bare-post removal, no-op

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/admin.py

@@ -379,6 +379,22 @@ async def trigger_dedup_videos():
     return jsonify({"task_id": async_result.id, "status": "queued"}), 202
 
 
+@admin_bp.route("/maintenance/purge-gated-previews", methods=["POST"])
+async def trigger_purge_gated_previews():
+    """Cleanup (#874 follow-up). Body {"dry_run": bool}: dry_run=true previews how
+    many blurred locked-preview images (grabbed from tier-gated Patreon posts
+    before the fix) would be removed WITHOUT deleting; dry_run=false applies it.
+    Re-walks every enabled Patreon source read-only and matches by content hash, so
+    real content downloaded when access existed is provably spared. Returns the
+    Celery task id — poll /maintenance/task-result/<id> for the summary."""
+    from ..tasks.admin import purge_gated_previews_task
+
+    body = await request.get_json(silent=True) or {}
+    dry_run = bool(body.get("dry_run", True))  # default to the SAFE preview
+    async_result = purge_gated_previews_task.delay(dry_run=dry_run)
+    return jsonify({"task_id": async_result.id, "status": "queued"}), 202
+
+
 @admin_bp.route("/maintenance/task-result/<task_id>", methods=["GET"])
 async def maintenance_task_result(task_id: str):
     """Poll a maintenance Celery task's result (the summary dict it returns).