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-platform/Cargo.toml

@@ -0,0 +1,36 @@
+[package]
+name = "tavolo-platform"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Game-independent half of the tavolo platform: lobby, live play, history, ratings"
+
+[dependencies]
+async-trait.workspace = true
+axum.workspace = true
+base64.workspace = true
+chrono.workspace = true
+futures-util.workspace = true
+jsonwebtoken.workspace = true
+rand.workspace = true
+redis.workspace = true
+regex.workspace = true
+reqwest.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+sha2.workspace = true
+sqlx.workspace = true
+thiserror.workspace = true
+tokio.workspace = true
+tower.workspace = true
+tracing.workspace = true
+url.workspace = true
+uuid.workspace = true
+
+[dev-dependencies]
+axum = { workspace = true }
+futures-util = { workspace = true }
+http-body-util = "0.1"
+tokio = { workspace = true }
+tokio-tungstenite = "0.24"
+tower = { workspace = true }