Stage 12: observability & performance (OTel/OTLP, domain metrics, guest GC)

- pkg/telemetry: shared OTel provider bootstrap (none/stdout/otlp + W3C
  propagators + Go runtime metrics); backend/internal/telemetry becomes a thin
  facade keeping its gin middleware.
- Telemetry parity: gateway and the Telegram connector gain telemetry runtimes
  and config (GATEWAY_/TELEGRAM_ SERVICE_NAME + OTEL_*); otelgrpc instruments the
  backend push server, the gateway's backend+connector clients and the connector
  server. Default exporter stays none (collector/dashboards are Stage 14).
- Operational metrics (variant attribute on game-scoped ones): game_replay_duration,
  game_move_validate_duration, games_started_total, games_abandoned_total,
  game_cache_active, chat_messages_total{kind}, gateway edge_request_duration.
  Wired via the SetMetrics setter pattern (default no-op meter).
- TODO-3: account.GuestReaper deletes guests with no game seat past
  BACKEND_GUEST_RETENTION (default 30d, swept every BACKEND_GUEST_REAP_INTERVAL).
- Tests: pkg/telemetry exporter selection; game/social/edge metric recording via
  a manual reader; config (otlp accepted, guest knobs); inttest guest reaper.
- Docs: PLAN.md re-scopes Stage 12 and adds Stage 13 (alphabet-on-wire) + Stage 14
  (CI/deploy) with the agreed dictionary-versioning resolution; ARCHITECTURE 11/13,
  TESTING, the three READMEs and FUNCTIONAL(+ru) updated.

docs/FUNCTIONAL.md

@@ -29,7 +29,8 @@ session token; the backend resolves it to an internal `user_id`. A **Telegram Mi
 App** launch authenticates from the platform's signed `initData`, themes the UI to
 the Telegram colours, and — on first contact — seeds the new account's interface
 language from the Telegram client. Guests are session-only with restricted features
-(auto-match only; no friends, stats or history). While the app is open the client
+(auto-match only; no friends, stats or history); an abandoned guest that never
+joined a game and has been idle past the retention window is garbage-collected. While the app is open the client
 keeps a live stream and receives in-app updates in real time — the opponent's move,
 your turn, chat, nudges and a found match. When the app is **closed**, the chosen
 out-of-app events (your turn, nudge, a found match, an invitation or friend request)