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/Dockerfile

@@ -57,8 +57,12 @@ RUN --mount=type=cache,target=/root/.cache/pip \
 
 COPY server/pyproject.toml server/README.md ./
 COPY server/src/ ./src/
+# Sibling distributions, installed from the local tree (PEP 420 namespace
+# packages sharing the `tavolo` namespace with the app above). --no-deps:
+# every third-party dependency is already pinned in requirements.txt.
+COPY server/packages/ ./packages/
 RUN --mount=type=cache,target=/root/.cache/pip \
-    /opt/venv/bin/pip install .
+    /opt/venv/bin/pip install --no-deps ./packages/tavolo-platform ./packages/tavolo-scopone .
 # aerich migration files are a release artifact: the db-migrate compose
 # service runs `aerich upgrade` from this image before the app starts.
 COPY server/migrations/ ./migrations/