Export app, process and log telemetry when OTEL_ENABLED is set

Adds tavolo.active_users and tavolo.ongoing_games gauges, process/CPython
metrics via opentelemetry-instrumentation-system-metrics and OTLP log
export. The game store tracks ongoing matches (Redis sorted set, in-memory
set) and the session stores expose live counts for the gauges; a poller
refreshes them since metric callbacks cannot await the stores.

docker-compose.yml

@@ -122,7 +122,8 @@ services:
       CORS_ALLOW_CREDENTIALS: ${CORS_ALLOW_CREDENTIALS:-}
       CORS_EXPOSE_HEADERS: ${CORS_EXPOSE_HEADERS:-}
       CORS_MAX_AGE: ${CORS_MAX_AGE:-}
-      # OpenTelemetry (kaya-otel): disabled unless OTEL_ENABLED is set.
+      # OpenTelemetry (kaya-otel + system-metrics): traces, 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).
       OTEL_ENABLED: ${OTEL_ENABLED:-}