feat(core): implement domain dataclasses for issue #5

- Create core/__init__.py
- Create core/models.py with all domain dataclasses:
  - Bounty (base class)
  - GroupBounty (extends Bounty)
  - PersonalBounty (extends Bounty)
  - TrackedBounty
  - GroupData
  - TrackingData
  - UserData
- Create tests/test_models.py with 15 passing tests

Fixes #5

core/models.py

@@ -0,0 +1,63 @@
+"""Domain dataclasses for JIGAIDO bounty tracker."""
+
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass
+class Bounty:
+    """Base bounty with common fields."""
+
+    id: int
+    text: Optional[str]
+    link: Optional[str]
+    due_date_ts: Optional[int]
+    created_at: int
+
+
+@dataclass
+class GroupBounty(Bounty):
+    """Bounty created in a group context."""
+
+    created_by_user_id: int
+
+
+@dataclass
+class PersonalBounty(Bounty):
+    """Bounty created in DM/personal context."""
+
+    pass
+
+
+@dataclass
+class TrackedBounty:
+    """A bounty that a user is tracking."""
+
+    bounty_id: int
+    created_at: int
+
+
+@dataclass
+class GroupData:
+    """All data for a group."""
+
+    group_id: int
+    bounties: list[GroupBounty]
+    next_id: int
+
+
+@dataclass
+class TrackingData:
+    """User tracking data within a group."""
+
+    user_id: int
+    tracked: list[TrackedBounty]
+
+
+@dataclass
+class UserData:
+    """All personal bounties for a user (DM mode)."""
+
+    user_id: int
+    bounties: list[PersonalBounty]
+    next_id: int