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/__init__.py

@@ -0,0 +1,21 @@
+"""Core domain models for JIGAIDO."""
+
+from core.models import (
+    Bounty,
+    GroupBounty,
+    PersonalBounty,
+    TrackedBounty,
+    GroupData,
+    TrackingData,
+    UserData,
+)
+
+__all__ = [
+    "Bounty",
+    "GroupBounty",
+    "PersonalBounty",
+    "TrackedBounty",
+    "GroupData",
+    "TrackingData",
+    "UserData",
+]