fix: RAG vector search SQL syntax error breaking assistant chat

- Use CAST(:embedding AS vector) instead of :embedding::vector to avoid
  SQLAlchemy named param conflict with PostgreSQL :: cast syntax
- Add db.rollback() before recording AI usage on failure to prevent
  InFailedSQLTransactionError cascade

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

backend/app/api/endpoints/assistant_chat.py

@@ -155,6 +155,7 @@ async def post_message(
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=str(e))
     except Exception as e:
         logger.exception("Assistant chat message failed: %s", e)
+        await db.rollback()
         await record_ai_usage(
             user_id=current_user.id,
             account_id=current_user.account_id,