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/Dockerfile

@@ -58,13 +58,13 @@ RUN --mount=type=cache,target=/root/.cache/pip \
  && /opt/venv/bin/pip install --upgrade pip \
  && /opt/venv/bin/pip install -r requirements.txt
 
-# The three sibling distributions, installed from the local tree (PEP 420
+# The four sibling distributions, installed from the local tree (PEP 420
 # namespace packages sharing the `tavolo` namespace). --no-deps: every
 # third-party dependency — including the tavolo-app `otel` extra — is
 # already pinned in requirements.txt.
 COPY server/packages/ ./packages/
 RUN --mount=type=cache,target=/root/.cache/pip \
-    /opt/venv/bin/pip install --no-deps ./packages/tavolo-platform ./packages/tavolo-scopone ./packages/tavolo-app
+    /opt/venv/bin/pip install --no-deps ./packages/tavolo-platform ./packages/tavolo-scopone ./packages/tavolo-briscola ./packages/tavolo-app
 # server/pyproject.toml only carries the [tool.aerich] section (the
 # db-migrate compose service runs `aerich upgrade` with working_dir=/app,
 # next to ./migrations).