feat: storage management — disk usage, cleanup API, scheduled cleanup task

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/tasks/cleanup.py

@@ -0,0 +1,10 @@
+from app.tasks.celery_app import celery_app
+from app.tasks.transcription import SyncSession
+from app.services.storage import cleanup_old_data
+
+
+@celery_app.task(name="app.tasks.cleanup.cleanup_old_data")
+def cleanup_old_data_task(retention_days: int | None = None):
+    with SyncSession() as db:
+        result = cleanup_old_data(db, retention_days)
+    return result