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-scopone/README.md

@@ -0,0 +1,36 @@
+# tavolo-scopone
+
+Scopone scientifico — the four-player, fixed-partnership Italian card
+game — as a [`tavolo-platform`](../tavolo-platform/README.md) game
+implementation.
+
+## Contents
+
+- `state.py` — `ScoponeState` (pure game data: phases, players, hands,
+  table, scores, deadlines), `PlayerState`, `Card`, `Move`, with JSON
+  (de)serialization. No session envelope, no transport, no I/O.
+- `engine.py` — the pure rules engine: deck, legal captures, plays,
+  auto-play, hand scoring (carte, denara, settebello, primiera, scope,
+  napola), hand-end acknowledgements. Deterministic and I/O-free apart
+  from logging, so the whole rule set is unit-testable.
+- `errors.py` — scopone-specific errors (`CardNotInHand`); every other
+  failure mode is a shared `tavolo.platform.errors` subclass.
+- `plugin.py` — `ScoponeEngine(GameEngine)`: the platform-facing adapter.
+  It translates create/join/websocket actions/deadlines into rules-engine
+  calls and back, validates the `target_score`/`napola` creation options,
+  and extracts the `MatchResult` (teams, winner, per-player scores and the
+  match summary persisted as the match's JSON `result`).
+
+Timeouts are constructor arguments (`turn_timeout_seconds`,
+`hand_ack_timeout_seconds`), wired from the environment by the
+application composition root.
+
+## Development (from `server/`)
+
+```sh
+.venv/bin/python -m unittest discover -s packages/tavolo-scopone/tests
+.venv/bin/python -m mypy -p tavolo.scopone
+```
+
+`test_engine.py` covers the pure rules; `test_plugin.py` covers the
+platform contract (actions, deadlines, serialization, results).