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.

server/packages/tavolo-app/pyproject.toml

@@ -35,6 +35,7 @@ dev = [
 ]
 otel = [
     "kaya-otel>=0.0.4",
+    "opentelemetry-instrumentation-system-metrics>=0.66b0",
 ]
 
 [tool.setuptools.packages.find]