feat: add hand detail API and enrich hand summary fields

- HandSummary: add hole_cards, starting_stacks, ending_stacks, pot_contributions
- Engine: capture all players' hole cards (not just showdown), pre/post hand stacks, per-level pot contributions
- Server: new GET /game/<game_id>/hands/<hand_number> route
- Service: add get_hand_state() method
- Tests: add ServerTests for new endpoint, update existing tests
- Existing GET /game/<game_id> auto-inherits new fields via shared to_dict()

tests/test_service.py

@@ -40,8 +40,16 @@ class ServiceTests(unittest.TestCase):
 
         hands = manager.run_hands(game.game_id, count=1)
 
+        state = manager.get_game_state("demo")
+        hand = manager.get_hand_state("demo", 1)
+
         self.assertEqual(len(hands), 1)
-        self.assertEqual(manager.get_game_state("demo")["hand_number"], 1)
+        self.assertEqual(state["hand_number"], 1)
+        self.assertEqual(hand, state["hands"][0])
+        self.assertIn("hole_cards", hand)
+        self.assertIn("starting_stacks", hand)
+        self.assertIn("ending_stacks", hand)
+        self.assertIn("pot_contributions", hand)
 
     def test_get_game_state_does_not_block_during_run(self) -> None:
         manager = GameManager()