Rewrite the backend in Rust with Axum

Replace the Python/kaya server in server/ with a Rust Cargo workspace of
three crates mirroring the old layout: tavolo-platform (lobby API, live-game
store, websocket endpoint, deadline scheduler, stats/Elo, OIDC sessions),
tavolo-scopone (pure scopone scientifico rules plus the platform adapter)
and tavolo-app (composition root and the `tavolo` binary with
serve/migrate/backfill-elo).

The HTTP and WebSocket contracts, error messages, Redis key layout and
Postgres schema are unchanged, so the existing web/ frontend and in-flight
games keep working. Notable differences: sessions are JSON under
tavolo:session: (Python pickle sessions invalidate once), LOGGING_CONFIG is
ignored in favour of RUST_LOG, DATABASE_URL must be Postgres, sqlx migrations
replace aerich, and OpenTelemetry exports Rust process metrics instead of
CPython gc metrics (no Postgres/Redis client instrumentation).

Docker Compose and the Kubernetes manifest now run the single Rust image
(embedded migrations, /static and /assets served by the app); the release CI
keeps building server/Dockerfile. The full test suite (scopone rules,
platform store/deadlines/stats/websockets, OIDC login flow, scopone e2e) is
ported and runs with cargo test --workspace; Redis/Postgres integration tests
are gated on TEST_REDIS_URL/TEST_DATABASE_URL.

server/crates/tavolo-scopone/src/errors.rs

@@ -0,0 +1,15 @@
+//! Scopone scientifico errors.
+//!
+//! Most failure modes are game-independent and live in
+//! [`tavolo_platform::errors`]; only genuinely scopone-specific errors are
+//! defined here. The Rust platform folds every game error into a single
+//! [`GameError`] value carrying an `"illegal_move"` code, so a Python-style
+//! exception hierarchy is unnecessary; this module keeps named constructors so
+//! call sites (and tests) stay readable.
+
+use tavolo_platform::errors::GameError;
+
+/// The played card is not held by the player.
+pub fn card_not_in_hand(code: &str) -> GameError {
+    GameError::illegal_move(format!("card {code} is not in your hand"))
+}