update rooms.py and main.py

backend/rooms.py

@@ -325,4 +325,15 @@ def remove_room_member(room_id, user_id):
         (room_id, user_id)
     )
 
-    return True
+    return True
+
+def update_member_role(room_id, user_id, role):
+    execute(
+        """
+        UPDATE room_members
+        SET role = ?
+        WHERE room_id = ?
+        AND user_id = ?
+        """,
+        (role, room_id, user_id),
+    )