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.

web/style.css

@@ -1,4 +1,4 @@
-/* Scopone scientifico frontend — game table, cards and scoring summary.
+/* tavolo frontend — game tables, cards and scoring summaries.
  *
  * The felt palette and the Bootstrap theme overrides live in theme.css,
  * which is loaded before this file. */
@@ -474,3 +474,69 @@ body {
     font-size: 1.25rem;
     font-weight: 700;
 }
+
+/* ---------- briscola ---------- */
+
+.briscola-badge {
+    background: var(--panel-light);
+    color: var(--accent);
+    font-weight: 700;
+    border-radius: 999px;
+    padding: 0.15rem 0.6rem;
+    border: 1px solid var(--accent);
+}
+
+/* The face-down stock with the face-up trump card beside it. */
+.stock {
+    display: flex;
+    align-items: center;
+    gap: 0.75rem;
+}
+
+.stock-pile {
+    display: flex;
+}
+
+.playing-card.stock-card {
+    height: 48px;
+    margin-left: -34px;
+}
+
+.playing-card.stock-card:first-child {
+    margin-left: 0;
+}
+
+.playing-card.briscola-card {
+    height: 64px;
+    outline: 2px solid var(--accent);
+    outline-offset: 2px;
+}
+
+.stock-count {
+    color: var(--muted);
+    font-size: 0.8rem;
+    font-variant-numeric: tabular-nums;
+}
+
+/* Current-trick cards with the player's name above each. */
+.trick-slot {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    gap: 0.15rem;
+}
+
+.trick-name {
+    font-size: 0.7rem;
+    color: var(--muted);
+}
+
+/* Two-player table: opponent on top, player at the bottom. */
+.table-grid.duo {
+    grid-template-columns: 1fr;
+    grid-template-rows: auto 1fr auto;
+    grid-template-areas:
+        "top"
+        "center"
+        "bottom";
+}