feat(thumb-backfill): /api/thumbnails/backfill endpoint — POST → 202 + celery_task_id

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/thumbnails.py

@@ -0,0 +1,13 @@
+"""Thumbnail admin API: backfill trigger."""
+
+from quart import Blueprint, jsonify
+
+thumbnails_bp = Blueprint("thumbnails", __name__, url_prefix="/api/thumbnails")
+
+
+@thumbnails_bp.route("/backfill", methods=["POST"])
+async def trigger_backfill():
+    from ..tasks.thumbnail import backfill_thumbnails
+
+    r = backfill_thumbnails.delay()
+    return jsonify({"celery_task_id": r.id}), 202