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/README.md

@@ -11,6 +11,10 @@ The deployable tavolo application: the composition root wiring
   `ScoponeEngine` with its configured timeouts, and exposes
   `tavolo.app:app` for granian.
 - `config.py` — environment-driven frozen `Settings`.
+- `telemetry.py` — the optional OpenTelemetry wiring beyond kaya-otel:
+  meter/logger providers, the `tavolo.active_users`/`tavolo.ongoing_games`
+  gauges (polled from the stores), process/CPython metrics via
+  `opentelemetry-instrumentation-system-metrics` and OTLP log export.
 - `aerich_config.py` — Tortoise ORM configuration consumed by the aerich
   CLI (migrations live in `server/migrations/`; aerich itself is
   configured via the `[tool.aerich]` section of `server/pyproject.toml`).
@@ -23,6 +27,7 @@ The deployable tavolo application: the composition root wiring
 .venv/bin/python -m mypy --namespace-packages --explicit-package-bases \
   src/tavolo/app.py \
   src/tavolo/config.py \
+  src/tavolo/telemetry.py \
   src/tavolo/static.py \
   src/tavolo/aerich_config.py \
   src/tavolo/logging_config.py