feat: switch admin identification from user_id to username

- Replace admin_user_ids (list[int]) with admin_usernames (list[str])
- Update all service methods to use username for permission checks
- Add delete button to bot responses for message cleanup
- Update tests to match new implementation

Note: Breaking change - existing data files need fresh start

adapters/storage/json_file.py

@@ -67,7 +67,7 @@ class JsonFileRoomStorage:
             bounties=bounties,
             next_id=data["next_id"],
             timezone=data.get("timezone"),
-            admin_user_ids=data.get("admin_user_ids", []),
+            admin_usernames=data.get("admin_usernames", []),
         )
 
     def save(self, room_data: RoomData) -> None:
@@ -76,7 +76,7 @@ class JsonFileRoomStorage:
             "room_id": room_data.room_id,
             "next_id": room_data.next_id,
             "timezone": room_data.timezone,
-            "admin_user_ids": room_data.admin_user_ids or [],
+            "admin_usernames": room_data.admin_usernames or [],
             "bounties": [
                 {
                     "id": b.id,