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/TESTING.md

@@ -93,6 +93,16 @@ tests or touching CI.
   dictionary-change pipeline** (file → resolve with a disposition → pending change → mark
   applied), the admin **list/count** read queries, and the **/_gm console over HTTP**
   (pages render; a resolve POST needs a same-origin header).
+- **Observability & performance** *(Stage 12)* — `pkg/telemetry` unit-tests the exporter
+  selection (`none`/`stdout`/`otlp` build providers; OTLP constructs with no collector;
+  the nil-runtime fallback). The domain metrics are exercised through a manual
+  `sdkmetric` reader: `backend/internal/game` and `…/social` assert the counters and
+  histograms record with the right `variant`/`kind` attributes, and
+  `gateway/internal/connectsrv` asserts `edge_request_duration` by `message_type`/
+  `result`. Config tests cover the new telemetry env vars (backend/gateway/connector —
+  `otlp` now accepted, an unsupported exporter rejected) and the guest-reaper knobs.
+  Postgres-backed `inttest` drives the **guest reaper** end to end (an abandoned guest is
+  reaped; a too-young guest, a seated guest and a durable account are kept).
 
 ## Principles