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

tests/test_models.py

@@ -122,7 +122,7 @@ class TestRoomData:
         assert rd.bounties == []
         assert rd.next_id == 1
         assert rd.timezone is None
-        assert rd.admin_user_ids == []
+        assert rd.admin_usernames == []
 
     def test_create_dm_room_data(self):
         rd = RoomData(
@@ -134,7 +134,7 @@ class TestRoomData:
         assert rd.bounties == []
         assert rd.next_id == 1
         assert rd.timezone is None
-        assert rd.admin_user_ids == []
+        assert rd.admin_usernames == []
 
     def test_room_data_with_bounties(self):
         b = Bounty(
@@ -156,18 +156,18 @@ class TestRoomData:
             bounties=[],
             next_id=1,
             timezone="Asia/Jakarta",
-            admin_user_ids=[123, 456],
+            admin_usernames=["alice", "bob"],
         )
         assert rd.timezone == "Asia/Jakarta"
-        assert rd.admin_user_ids == [123, 456]
+        assert rd.admin_usernames == ["alice", "bob"]
 
-    def test_room_data_admin_user_ids_defaults_to_empty_list(self):
+    def test_room_data_admin_usernames_defaults_to_empty_list(self):
         rd = RoomData(
             room_id=-1001,
             bounties=[],
             next_id=1,
         )
-        assert rd.admin_user_ids == []
+        assert rd.admin_usernames == []
 
 
 class TestTrackingData: