Split backend into tavolo-platform, tavolo-scopone and tavolo-app packages

Move the game-independent machinery (lobby, live-game store, websocket,
deadline scheduler, match history, leaderboards) into a new
tavolo-platform distribution behind a GameEngine contract, the scopone
scientifico rules plus a platform adapter into tavolo-scopone, and keep
only the composition root in tavolo-app. The three distributions share
the tavolo namespace (PEP 420, kaya-style monorepo).

Match history becomes fully generic: Match carries the engine's result
JSON and MatchPlayer points/details instead of scopone-shaped team
columns (migration 3 backfills existing rows). Lobby creation takes an
opaque per-game options object and websocket actions dispatch to the
session's engine.

Tests: platform suite runs against a DummyEngine toy game, scopone
keeps the rules tests plus new adapter tests, server/tests covers the
wired stack end to end (194 tests, was 143).

server/packages/tavolo-platform/src/tavolo/platform/mixin.py

@@ -0,0 +1,50 @@
+"""The :class:`PlatformMixin`: mounts the whole platform on a kaya app.
+
+``Platform`` bundles the collaborators every platform endpoint needs —
+the game registry, the live-game store, the deadline scheduler and the
+OIDC mixin used for authentication — so route and websocket modules
+never reach for module-global state. :class:`PlatformMixin` is a plain
+:class:`~kaya.core.KayaMixin`: applying it registers the lobby, stats
+and health HTTP routes plus the live-play websocket endpoint, and the
+app stays a :class:`~kaya.core.KayaApp` (both ASGI and RSGI keep
+working).
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from kaya.core import KayaApp, KayaMixin
+from kaya.oidc import OIDCMixin
+
+from .deadlines import DeadlineScheduler
+from .registry import GameRegistry
+from .store import GameStore
+
+
+@dataclass(frozen=True)
+class Platform:
+    """The collaborators shared by all platform endpoints."""
+
+    registry: GameRegistry
+    game_store: GameStore
+    scheduler: DeadlineScheduler
+    oidc: OIDCMixin
+
+
+class PlatformMixin(KayaMixin):
+    """Register the game-independent routes and the websocket endpoint."""
+
+    def __init__(self, platform: Platform) -> None:
+        self.platform = platform
+
+    def apply(self, app: KayaApp) -> None:
+        # Imported here so module import order stays acyclic: the route
+        # modules reference Platform only for typing.
+        from . import ws
+        from .routes import games, health, me, stats
+
+        health.register(app, self.platform)
+        me.register(app, self.platform)
+        games.register(app, self.platform)
+        stats.register(app, self.platform)
+        ws.register(app, self.platform)