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-app/README.md

@@ -2,14 +2,15 @@
 
 The deployable tavolo application: the composition root wiring
 [`tavolo-platform`](../tavolo-platform/README.md) to the
-[`tavolo-scopone`](../tavolo-scopone/README.md) game.
+[`tavolo-scopone`](../tavolo-scopone/README.md) and
+[`tavolo-briscola`](../tavolo-briscola/README.md) games.
 
 ## Contents
 
 - `app.py` — assembles the `KayaApp` (session/OIDC/Tortoise/OpenAPI
   mixins plus `PlatformMixin` and the deadline scheduler), registers the
-  `ScoponeEngine` with its configured timeouts, and exposes
-  `tavolo.app:app` for granian.
+  `ScoponeEngine`, `BriscolaEngine` and `BriscolaQuattroEngine` with
+  their configured timeouts, and exposes `tavolo.app:app` for granian.
 - `config.py` — environment-driven frozen `Settings`.
 - `telemetry.py` — the optional OpenTelemetry wiring beyond kaya-otel:
   meter/tracer/logger providers, the