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.

core/models.py

@@ -9,6 +9,9 @@ class Bounty:
 
     The created_by_user_id field always refers to the user who created the bounty.
     It does NOT indicate whether the bounty is a group or personal bounty.
+
+    The deleted_at field indicates soft-delete: None means not deleted,
+    a value means deleted at that Unix timestamp.
     """
 
     id: int
@@ -17,6 +20,8 @@ class Bounty:
     due_date_ts: int | None
     created_at: int
     created_by_user_id: int
+    deleted_at: int | None = None
+    created_by_username: str | None = None
 
 
 @dataclass
@@ -37,11 +42,20 @@ class RoomData:
 
     The room_id can be negative for Telegram groups or positive for DMs.
     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.
     """
 
     room_id: int
     bounties: list[Bounty]
     next_id: int
+    timezone: str | None = None
+    admin_user_ids: list[int] | None = None
+
+    def __post_init__(self):
+        if self.admin_user_ids is None:
+            self.admin_user_ids = []
 
 
 @dataclass