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).

README.md

@@ -5,6 +5,9 @@ scientifico** — the four-player, fixed-partnership Italian card game:
 
 - **`server/`** — backend: Python + [kaya](https://github.com/woggioni/kaya)
   framework, OIDC login, live games in Redis, match statistics in Postgres.
+  A monorepo of three distributions sharing the `tavolo` namespace: the
+  game-independent `tavolo-platform`, the scopone game `tavolo-scopone`,
+  and the `tavolo-app` composition root wiring them together.
   See [server/README.md](server/README.md).
 - **`web/`** — frontend: Rust + [Sycamore](https://sycamore.dev) compiled to
   WebAssembly, built with [Trunk](https://trunkrs.dev). Card images are the
@@ -53,9 +56,10 @@ Backend (from `server/`):
 cd server
 python3 -m venv .venv
 .venv/bin/pip install -r requirements.txt
-.venv/bin/pip install -e '.[dev]'
+.venv/bin/pip install -e ./packages/tavolo-platform -e ./packages/tavolo-scopone -e '.[dev]'
 .venv/bin/python -m unittest discover -s tests -t .
-.venv/bin/mypy src
+.venv/bin/python -m unittest discover -s packages/tavolo-platform/tests
+.venv/bin/python -m unittest discover -s packages/tavolo-scopone/tests
 ```
 
 Frontend (from `web/`), with the backend running on :8080: