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.

docker-compose.yml

@@ -61,16 +61,14 @@ services:
       timeout: 3s
       retries: 10
 
-  # One-shot: applies pending aerich migrations before the app starts
-  # (build cache makes the second build of the same Dockerfile instant).
+  # One-shot: applies pending sqlx migrations before the app starts (the
+  # migrations are embedded in the image; build cache makes the second build
+  # of the same Dockerfile instant).
   db-migrate:
     build:
       context: .
       dockerfile: server/Dockerfile
-    # aerich reads [tool.aerich] from /app/pyproject.toml (default config
-    # file) and finds migrations in ./migrations relative to working_dir.
-    working_dir: /app
-    command: ["aerich", "upgrade"]
+    command: ["tavolo", "migrate"]
     environment:
       DATABASE_ENGINE: postgres
       DATABASE_HOST: postgres
@@ -124,11 +122,9 @@ services:
       CORS_ALLOW_CREDENTIALS: ${CORS_ALLOW_CREDENTIALS:-}
       CORS_EXPOSE_HEADERS: ${CORS_EXPOSE_HEADERS:-}
       CORS_MAX_AGE: ${CORS_MAX_AGE:-}
-      # OpenTelemetry (kaya-otel + contrib instrumentations): traces
-      # (HTTP/WebSocket, Postgres, Redis), metrics and logs via OTLP/HTTP,
-      # disabled unless OTEL_ENABLED is set.
-      # The image bundles the otel extra (see packages/tavolo-app/pyproject.toml
-      # and server/requirements.txt).
+      # OpenTelemetry (compiled into the image): traces (HTTP/WebSocket),
+      # metrics (process + tavolo gauges) and logs via OTLP/HTTP, disabled
+      # unless OTEL_ENABLED is set.
       OTEL_ENABLED: ${OTEL_ENABLED:-}
       OTEL_SERVICE_NAME: ${OTEL_SERVICE_NAME:-}
       OTEL_EXPORTER_OTLP_ENDPOINT: ${OTEL_EXPORTER_OTLP_ENDPOINT:-}