Preview a match by join code before joining

GET /api/games/preview returns the lobby behind a code plus the game's
name and engine-echoed options. The lobby debounces the lookup as a
complete code is typed and shows the game, seated players, open seats
and option badges rendered from the game's schema, disabling Join when
the lobby is full or the match has already started.

server/README.md

@@ -191,6 +191,7 @@ All endpoints except `/api/health`, `/api/docs`, `/api/openapi.json`,
 | `GET` | `/api/game-types` | The games the platform can host (for the creation dropdown), each with its `options_schema` |
 | `POST` | `/api/games` | Create a lobby game. Optional body `{"game_type": "scopone_scientifico", "options": {"target_score": 11, "napola": true, "asso_pigliatutto": false}}`. Returns the lobby (`{id, join_code, players, seats_open, ...}`) |
 | `POST` | `/api/games/join` | Join with `{"code": "ABC123"}`. The last player triggers the start and gets the game view back |
+| `GET` | `/api/games/preview?code=ABC123` | Pre-join summary of the lobby behind a code: game name/description, seated players, open seats and the engine-echoed options. `404` when the code is unknown |
 | `GET` | `/api/games/{id}` | Personalized snapshot (only your own hand is visible) |
 | `GET` | `/api/me/matches` | Cursor-paginated match history with the engine's `result` and per-player Elo deltas (`?limit=&cursor=&game_type=`) |
 | `GET` | `/api/me/ratings` | The caller's Elo rating per game type |