test: add unit tests for models, payout logic, and room lock

- test_models.py: 10 tests for Room/Horse/Bet/RaceResult dataclasses
- test_payout_logic.py: 12 tests for payout formula (max+round)
- test_room_store_lock.py: 5 tests for get_lock() setdefault pattern
- All 34 tests pass in 0.27s

tests/conftest.py

@@ -0,0 +1,8 @@
+"""Test configuration - add project root to sys.path."""
+import sys
+from pathlib import Path
+
+# Add project root so `danding_bot` can be imported
+project_root = Path(__file__).parent.parent
+if str(project_root) not in sys.path:
+    sys.path.insert(0, str(project_root))