fix: serialize SQLite close with cancelled worker transactions

control/accounts.py

@@ -103,7 +103,11 @@ class Accounts:
                     os.chmod(companion, 0o600)
 
     def close(self):
-        self.db.close()
+        # Cancelling an asyncio.to_thread caller does not stop its SQLite worker.
+        # ChatStore/FeatureStore share this connection and lock; wait for any
+        # in-flight transaction before closing the native connection.
+        with self.lock:
+            self.db.close()
 
     def users(self):
         with self.lock: