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.

platform/telegram/internal/config/config.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"strconv"
 	"strings"
+
+	pkgtel "scrabble/pkg/telemetry"
 )
 
 // Config is the Telegram connector's runtime configuration, read from the
@@ -34,6 +36,8 @@ type Config struct {
 	GameChannelID int64
 	// LogLevel is the zap log level (TELEGRAM_LOG_LEVEL, default info).
 	LogLevel string
+	// Telemetry configures the OpenTelemetry providers (shared bootstrap).
+	Telemetry pkgtel.Config
 }
 
 // Load reads the connector configuration from the environment, applying defaults
@@ -47,6 +51,11 @@ func Load() (Config, error) {
 		TestEnv:    os.Getenv("TELEGRAM_TEST_ENV") == "true",
 		LogLevel:   envOr("TELEGRAM_LOG_LEVEL", "info"),
 	}
+	tel := pkgtel.DefaultConfig("scrabble-telegram")
+	tel.ServiceName = envOr("TELEGRAM_SERVICE_NAME", tel.ServiceName)
+	tel.TracesExporter = envOr("TELEGRAM_OTEL_TRACES_EXPORTER", tel.TracesExporter)
+	tel.MetricsExporter = envOr("TELEGRAM_OTEL_METRICS_EXPORTER", tel.MetricsExporter)
+	cfg.Telemetry = tel
 	if cfg.BotToken == "" {
 		return Config{}, fmt.Errorf("config: TELEGRAM_BOT_TOKEN is required")
 	}
@@ -60,6 +69,9 @@ func Load() (Config, error) {
 		}
 		cfg.GameChannelID = id
 	}
+	if err := cfg.Telemetry.Validate(); err != nil {
+		return Config{}, fmt.Errorf("config: %w", err)
+	}
 	return cfg, nil
 }