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

core/models.py

@@ -44,18 +44,18 @@ class RoomData:
     The next_id field is used to generate unique bounty IDs within this room.
 
     The timezone field stores the room's timezone (e.g., "Asia/Jakarta"), default UTC+0.
-    The admin_user_ids field lists users who have admin privileges in this room.
+    The admin_usernames field lists usernames who have admin privileges in this room.
     """
 
     room_id: int
     bounties: list[Bounty]
     next_id: int
     timezone: str | None = None
-    admin_user_ids: list[int] | None = None
+    admin_usernames: list[str] | None = None
 
     def __post_init__(self):
-        if self.admin_user_ids is None:
-            self.admin_user_ids = []
+        if self.admin_usernames is None:
+            self.admin_usernames = []
 
 
 @dataclass