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/cmd/backend/main.go

@@ -80,6 +80,9 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 			logger.Warn("telemetry shutdown", zap.Error(err))
 		}
 	}()
+	if err := tel.StartRuntimeMetrics(); err != nil {
+		logger.Warn("telemetry: start runtime metrics", zap.Error(err))
+	}
 
 	db, err := postgres.Open(ctx, cfg.Postgres,
 		postgres.WithTracerProvider(tel.TracerProvider()),
@@ -131,10 +134,19 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	accounts := account.NewStore(db)
 	games := game.NewService(game.NewStore(db), accounts, registry, cfg.Game, logger)
 	games.SetNotifier(hub)
+	games.SetMetrics(tel.MeterProvider().Meter("scrabble/backend/game"))
 	go games.RunSweeper(ctx, cfg.Game.TimeoutSweepInterval)
 	logger.Info("game turn-timeout sweeper started",
 		zap.Duration("interval", cfg.Game.TimeoutSweepInterval))
 
+	// Stage 12 TODO-3: reap abandoned guest accounts (no game seat, account age past
+	// the retention window). Dependent rows fall away via ON DELETE CASCADE.
+	guestReaper := account.NewGuestReaper(accounts, cfg.GuestRetention, logger)
+	go guestReaper.Run(ctx, cfg.GuestReapInterval)
+	logger.Info("guest reaper started",
+		zap.Duration("interval", cfg.GuestReapInterval),
+		zap.Duration("retention", cfg.GuestRetention))
+
 	// Stage 4 lobby & social domains. Their REST and stream surface is added with
 	// the gateway in Stage 6, so they are handed to the server (like the route
 	// groups) for the handlers to come.
@@ -145,6 +157,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	links := link.NewService(emails, accounts, accountmerge.NewMerger(db), sessions)
 	socialSvc := social.NewService(social.NewStore(db), accounts, games)
 	socialSvc.SetNotifier(hub)
+	socialSvc.SetMetrics(tel.MeterProvider().Meter("scrabble/backend/social"))
 
 	// Stage 5 robot opponent: provision its durable account pool (a hard startup
 	// dependency, like the dictionaries) and start its move driver. The matchmaker