Initial scopone scientifico backend

Multiplayer scopone scientifico backend on the kaya framework:

- OIDC login (kaya-oidc), session-backed WebSocket auth
- Pure rules engine (forced captures, scopa, primiera scoring) with
  full-match simulation tests
- Live game state in Redis (JSON + TTL, join codes, per-game locks,
  pub/sub state push); in-memory fallback for tests
- WebSocket /ws/games/{id} for real-time play; REST lobby endpoints
  (create/join/snapshot) with hidden-hand views
- Finished matches persisted to Postgres (Tortoise + aerich) for match
  history and leaderboard endpoints
- Docker Compose stack: postgres, redis, mock-oauth2-server, db-migrate, app
- 45 tests passing; mypy clean

src/scopa/game/errors.py

@@ -0,0 +1,42 @@
+"""Typed errors raised by the scopone engine.
+
+Route/WebSocket handlers translate these into 4xx responses or ``error``
+WebSocket messages; the engine itself stays transport-agnostic.
+"""
+from __future__ import annotations
+
+
+class GameError(Exception):
+    """Base class for every rule/validation failure."""
+
+
+class IllegalMove(GameError):
+    """The requested play violates the rules of scopone scientifico."""
+
+
+class NotYourTurn(GameError):
+    """A player attempted to play out of turn."""
+
+
+class CardNotInHand(GameError):
+    """The played card is not held by the player."""
+
+
+class GameNotStarted(GameError):
+    """An action was attempted before the game left the lobby."""
+
+
+class GameFinished(GameError):
+    """An action was attempted after the match ended."""
+
+
+class LobbyFull(GameError):
+    """A game already has four players."""
+
+
+class AlreadyJoined(GameError):
+    """A player tried to join a game they are already seated in."""
+
+
+class GameNotFound(GameError):
+    """No live game exists for the given id or join code."""