fix(#355 review round-2 F9-F11): register-gate test + shutdown idle-close + DB-path metrics gate

- F10 [stability]: closeMetricsServer() now calls server.closeIdleConnections()
  + server.unref() after server.close(). server.close()'s callback doesn't fire
  until keep-alive sockets drain, and the scraper (VictoriaMetrics/vmagent) holds
  an idle keep-alive socket — so onModuleDestroy's awaited close would hang until
  the scraper disconnects or the orchestrator SIGKILLs on the kill-grace window.
  closeIdleConnections() drops idle keep-alive sockets so shutdown completes
  immediately (Node 22, per the Dockerfile base).
- F9 [test]: client-telemetry.module.spec.ts pins the E1=B register() gate — the
  core of the "public endpoint OFF by default" decision: flag unset / any non-
  "true" value ("false"/""/"0"/…) → empty controllers+providers (route absent);
  "true"/"TRUE" → registers VitalsController + VitalsService. A flag-inversion or
  truthiness regression that reopened the anonymous disk-fill surface now fails.
- F11 [regression/perf]: the db_query_duration_seconds token work (firstSqlToken
  regex + Set lookup) is now gated on isMetricsEnabled() in database.module.ts, so
  a non-metrics deployment pays NOTHING per query (previously observeDbQuery
  no-op'd but the token was still computed on every query). Also hoisted the
  13-element known-token Set to a module const (KNOWN_SQL_TOKENS) so it's built
  once, not per query.

Gate: server tsc 0; metrics + vitals + client-telemetry suites pass (incl. the
new register-gate test).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/telemetry/client-telemetry.module.spec.ts

@@ -0,0 +1,47 @@
+import { ClientTelemetryModule } from './client-telemetry.module';
+import { VitalsController } from './vitals.controller';
+import { VitalsService } from './vitals.service';
+
+// The register() gate is the CORE of the maintainer's E1=B decision: the public,
+// unauthenticated /api/telemetry/vitals endpoint must be OFF by default, so a
+// self-host deploy has no anonymous disk-fill surface into `client_metrics`. A
+// regression that inverts the flag (or a truthiness bug where "" / "false"
+// registers the route) would silently reopen that surface — pin it here.
+describe('ClientTelemetryModule.register (E1=B gate)', () => {
+  const original = process.env.CLIENT_TELEMETRY_ENABLED;
+  afterEach(() => {
+    if (original === undefined) delete process.env.CLIENT_TELEMETRY_ENABLED;
+    else process.env.CLIENT_TELEMETRY_ENABLED = original;
+  });
+
+  it('OFF by default (flag unset) — no controller, no provider (endpoint absent)', () => {
+    delete process.env.CLIENT_TELEMETRY_ENABLED;
+    const mod = ClientTelemetryModule.register();
+    expect(mod.controllers).toEqual([]);
+    expect(mod.providers).toEqual([]);
+  });
+
+  it.each(['false', 'False', '0', '', 'yes', '1'])(
+    'stays OFF for non-"true" value %p (no route)',
+    (val) => {
+      process.env.CLIENT_TELEMETRY_ENABLED = val;
+      const mod = ClientTelemetryModule.register();
+      expect(mod.controllers).toEqual([]);
+      expect(mod.providers).toEqual([]);
+    },
+  );
+
+  it('ON only for "true" — registers VitalsController + VitalsService', () => {
+    process.env.CLIENT_TELEMETRY_ENABLED = 'true';
+    const mod = ClientTelemetryModule.register();
+    expect(mod.controllers).toContain(VitalsController);
+    expect(mod.providers).toContain(VitalsService);
+  });
+
+  it('ON is case-insensitive ("TRUE")', () => {
+    process.env.CLIENT_TELEMETRY_ENABLED = 'TRUE';
+    const mod = ClientTelemetryModule.register();
+    expect(mod.controllers).toContain(VitalsController);
+    expect(mod.providers).toContain(VitalsService);
+  });
+});