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/config/config_test.go

@@ -151,12 +151,54 @@ func TestLoadRejectsMalformedDuration(t *testing.T) {
 	}
 }
 
-// TestLoadRejectsUnsupportedExporter verifies that an exporter outside the MVP
-// set is rejected.
+// TestLoadRejectsUnsupportedExporter verifies that an exporter outside the
+// supported set is rejected.
 func TestLoadRejectsUnsupportedExporter(t *testing.T) {
 	t.Setenv("BACKEND_POSTGRES_DSN", testDSN)
-	t.Setenv("BACKEND_OTEL_TRACES_EXPORTER", "otlp")
+	t.Setenv("BACKEND_OTEL_TRACES_EXPORTER", "prometheus")
 	if _, err := Load(); err == nil {
 		t.Fatal("Load: expected an error for an unsupported exporter, got nil")
 	}
 }
+
+// TestLoadAcceptsOTLPExporter verifies that the otlp exporter is now accepted
+// (the collector is stood up with the deploy; the default stays none).
+func TestLoadAcceptsOTLPExporter(t *testing.T) {
+	t.Setenv("BACKEND_POSTGRES_DSN", testDSN)
+	t.Setenv("BACKEND_DICT_DIR", "/dict")
+	t.Setenv("BACKEND_OTEL_TRACES_EXPORTER", "otlp")
+	t.Setenv("BACKEND_OTEL_METRICS_EXPORTER", "otlp")
+	if _, err := Load(); err != nil {
+		t.Fatalf("Load with otlp exporters: %v", err)
+	}
+}
+
+// TestLoadGuestReaperDefaultsAndOverride covers the guest-reaper knobs: defaults
+// when unset, an override, and rejection of a non-positive value.
+func TestLoadGuestReaperDefaultsAndOverride(t *testing.T) {
+	t.Setenv("BACKEND_POSTGRES_DSN", testDSN)
+	t.Setenv("BACKEND_DICT_DIR", "/dict")
+
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.GuestReapInterval != defaultGuestReapInterval {
+		t.Errorf("GuestReapInterval = %s, want %s", c.GuestReapInterval, defaultGuestReapInterval)
+	}
+	if c.GuestRetention != defaultGuestRetention {
+		t.Errorf("GuestRetention = %s, want %s", c.GuestRetention, defaultGuestRetention)
+	}
+
+	t.Setenv("BACKEND_GUEST_RETENTION", "168h")
+	if c, err = Load(); err != nil {
+		t.Fatalf("Load (override): %v", err)
+	} else if c.GuestRetention != 168*time.Hour {
+		t.Errorf("GuestRetention = %s, want 168h", c.GuestRetention)
+	}
+
+	t.Setenv("BACKEND_GUEST_REAP_INTERVAL", "0s")
+	if _, err := Load(); err == nil {
+		t.Fatal("Load: expected an error for a non-positive reap interval, got nil")
+	}
+}