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.

backend/internal/telemetry/telemetry_test.go

@@ -12,9 +12,15 @@ func TestConfigValidate(t *testing.T) {
 	}
 
 	otlp := DefaultConfig()
-	otlp.TracesExporter = "otlp"
-	if err := otlp.Validate(); err == nil {
-		t.Error("otlp exporter must be rejected in the MVP set")
+	otlp.TracesExporter = ExporterOTLP
+	if err := otlp.Validate(); err != nil {
+		t.Errorf("otlp exporter must be accepted: %v", err)
+	}
+
+	bad := DefaultConfig()
+	bad.MetricsExporter = "prometheus"
+	if err := bad.Validate(); err == nil {
+		t.Error("unsupported exporter must be rejected")
 	}
 
 	noName := DefaultConfig()