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/__init__.py

@@ -38,6 +38,7 @@ def all_blueprints() -> list[Blueprint]:
     from .system_activity import system_activity_bp
     from .system_backup import system_backup_bp
     from .tags import tags_bp
+    from .thumbnails import thumbnails_bp
     return [
         api_bp,
         attachments_bp,
@@ -58,6 +59,7 @@ def all_blueprints() -> list[Blueprint]:
         allowlist_bp,
         aliases_bp,
         ml_admin_bp,
+        thumbnails_bp,
         sources_bp,
         platforms_bp,
         posts_bp,

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