Add briscola: classic two-player and briscola a quattro

New tavolo-briscola distribution implementing briscola on the platform:

- pure rules (state.py, engine.py): 3-card deals, a face-up trump that
  is the last card drawn, trump-first trick resolution (A > 3 > K > H >
  J > 7 > 6 > 5 > 4 > 2), 120 points per deal, 60-60 patta re-deals, a
  configurable target score (default 61), deal-end acknowledgements and
  turn auto-play;
- BriscolaEngine (briscola, 2 players) and BriscolaQuattroEngine
  (briscola_a_quattro, 4 players in fixed partnerships) adapters;
- registered in tavolo-app, packaged in the Docker image, documented.

Platform fixes surfaced by a two-player lobby:

- the websocket state and snapshot payloads now carry seats_open (they
  were only in the REST lobby payload), so lobbies with player counts
  other than four render correctly; seats_open is clamped at zero;
- the web client logs unparseable server messages instead of silently
  dropping them.

Web client: briscola table for both variants (stock, face-up trump,
trick, last-trick banner), deal summary modal and game-over overlay.

Tests: tavolo-briscola rules/adapter suites, app integration tests
(lobby, websocket play, turn timeout, deal-end acks, persistence), a
platform seats_open test and frontend model parsing tests.

server/packages/tavolo-platform/tests/test_ws.py

@@ -132,6 +132,33 @@ class WebSocketTest(unittest.TestCase):
                             self.assertEqual("state", update["type"])
                             self.assertEqual(1, update["game"]["plays"])
 
+    @async_test
+    async def test_state_reports_open_seats(self) -> None:
+        app, platform, _ = make_platform()
+        api_transport = ASGITransport(app=app)
+        async with AsyncClient(transport=api_transport, base_url="http://127.0.0.1") as client:
+            with oidc_user(platform.oidc, "alice"):
+                created = await client.post("/api/games", json={})
+            game_id = created.json()["id"]
+            code = created.json()["join_code"]
+
+            ws_transport = ASGIWebSocketTransport(app=app)
+            async with AsyncClient(transport=ws_transport, base_url="http://testserver") as ws_client:
+                with ws_users([make_user("alice")]):
+                    async with aconnect_ws(f"/ws/games/{game_id}", ws_client) as ws:
+                        first = await ws.receive_json()
+                        self.assertEqual(1, first["game"]["seats_open"])
+
+            # Once the second player joins the match starts and no seat is
+            # left open.
+            with oidc_user(platform.oidc, "bob"):
+                await client.post("/api/games/join", json={"code": code})
+            async with AsyncClient(transport=ws_transport, base_url="http://testserver") as ws_client:
+                with ws_users([make_user("alice")]):
+                    async with aconnect_ws(f"/ws/games/{game_id}", ws_client) as ws:
+                        first = await ws.receive_json()
+                        self.assertEqual(0, first["game"]["seats_open"])
+
     @async_test
     async def test_unknown_action_returns_error(self) -> None:
         app, platform, _ = make_platform()