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/src/pages/lobby.rs

@@ -20,14 +20,32 @@ use crate::model::{
 
 /// Used when the game-types fetch fails: match creation must still work.
 fn fallback_game_types() -> Vec<GameTypeInfo> {
-    vec![GameTypeInfo {
-        id: "scopone_scientifico".to_string(),
-        name: "Scopone scientifico".to_string(),
-        description: String::new(),
-        min_players: 4,
-        max_players: 4,
-        options_schema: serde_json::Value::Null,
-    }]
+    vec![
+        GameTypeInfo {
+            id: "scopone_scientifico".to_string(),
+            name: "Scopone scientifico".to_string(),
+            description: String::new(),
+            min_players: 4,
+            max_players: 4,
+            options_schema: serde_json::Value::Null,
+        },
+        GameTypeInfo {
+            id: "briscola".to_string(),
+            name: "Briscola".to_string(),
+            description: String::new(),
+            min_players: 2,
+            max_players: 2,
+            options_schema: serde_json::Value::Null,
+        },
+        GameTypeInfo {
+            id: "briscola_a_quattro".to_string(),
+            name: "Briscola a quattro".to_string(),
+            description: String::new(),
+            min_players: 4,
+            max_players: 4,
+            options_schema: serde_json::Value::Null,
+        },
+    ]
 }
 
 /// Human-readable label for a provider name ("google" -> "Google").
@@ -552,7 +570,7 @@ pub fn LobbyPage() -> View {
                             div(class="lobby-hero text-center") {
                                 h1(class="display-5 fw-bold") { "tavolo" }
                                 p(class="lead text-secondary mb-0") {
-                                    "Play scopone scientifico with friends."
+                                    "Play scopone scientifico and briscola with friends."
                                 }
                             }
                             div(class="row g-4") {