feat: add deleted_at, created_by_username to Bounty; timezone, admin_user_ids to RoomData

Issue #41: Model updates for Phase 2 features

Bounty model:
- Add deleted_at: int | None - timestamp when deleted (soft-delete)
- Add created_by_username: str | None - username for display purposes

RoomData model:
- Add timezone: str | None - room's timezone (e.g., "Asia/Jakarta")
- Add admin_user_ids: list[int] - list of admin user IDs

Storage adapter updated to handle new fields in load/save operations.
Tests added for new fields.

tests/test_models.py

@@ -28,6 +28,22 @@ class TestBounty:
         assert b.due_date_ts == 1735689600
         assert b.created_at == 1735603200
         assert b.created_by_user_id == 123
+        assert b.deleted_at is None
+        assert b.created_by_username is None
+
+    def test_bounty_with_new_fields(self):
+        b = Bounty(
+            id=1,
+            text="Fix the bug",
+            link="https://github.com/example/repo/issues/1",
+            due_date_ts=1735689600,
+            created_at=1735603200,
+            created_by_user_id=123,
+            deleted_at=1736200000,
+            created_by_username="johndoe",
+        )
+        assert b.deleted_at == 1736200000
+        assert b.created_by_username == "johndoe"
 
     def test_bounty_optional_fields_can_be_none(self):
         b = Bounty(
@@ -41,6 +57,8 @@ class TestBounty:
         assert b.text is None
         assert b.link is None
         assert b.due_date_ts is None
+        assert b.deleted_at is None
+        assert b.created_by_username is None
 
     def test_bounty_comparison_equal(self):
         b1 = Bounty(
@@ -103,6 +121,8 @@ class TestRoomData:
         assert rd.room_id == -1001
         assert rd.bounties == []
         assert rd.next_id == 1
+        assert rd.timezone is None
+        assert rd.admin_user_ids == []
 
     def test_create_dm_room_data(self):
         rd = RoomData(
@@ -113,6 +133,8 @@ class TestRoomData:
         assert rd.room_id == 123456
         assert rd.bounties == []
         assert rd.next_id == 1
+        assert rd.timezone is None
+        assert rd.admin_user_ids == []
 
     def test_room_data_with_bounties(self):
         b = Bounty(
@@ -128,6 +150,25 @@ class TestRoomData:
         assert rd.bounties[0].text == "Task"
         assert rd.bounties[0].created_by_user_id == 123
 
+    def test_room_data_with_new_fields(self):
+        rd = RoomData(
+            room_id=-1001,
+            bounties=[],
+            next_id=1,
+            timezone="Asia/Jakarta",
+            admin_user_ids=[123, 456],
+        )
+        assert rd.timezone == "Asia/Jakarta"
+        assert rd.admin_user_ids == [123, 456]
+
+    def test_room_data_admin_user_ids_defaults_to_empty_list(self):
+        rd = RoomData(
+            room_id=-1001,
+            bounties=[],
+            next_id=1,
+        )
+        assert rd.admin_user_ids == []
+
 
 class TestTrackingData:
     def test_create_tracking_data(self):