feat(phase-4): WS6 — Billing & Usage Metering (Stripe, free tier enforcement)

- DB migration 023: tenant_subscriptions and usage_events tables
- UsageMeteringMiddleware: in-memory counters, 60s flush to DB via UPSERT
- FreeTierEnforcementMiddleware: 10 agents / 1,000 calls/day limits, Redis cache
- UsageService: getDailyUsage and getActiveAgentCount
- BillingService: Stripe checkout sessions, webhook verification, subscription status
- POST /billing/checkout, POST /billing/webhook, GET /billing/usage endpoints
- BILLING_ENABLED=false disables enforcement without breaking metering
- Dashboard: Usage tab with Free Tier/Pro badges and metric cards
- 19 unit tests passing across billing services and middleware

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/metrics/registry.test.ts

@@ -19,6 +19,8 @@ import {
   rateLimitHitsTotal,
   dbPoolActiveConnections,
   dbPoolWaitingRequests,
+  tenantApiCallsTotal,
+  billingLimitRejectionsTotal,
 } from '../../../src/metrics/registry';
 
 describe('metricsRegistry', () => {
@@ -33,9 +35,9 @@ describe('metricsRegistry', () => {
     expect(metricsRegistry).not.toBe(register);
   });
 
-  it('contains exactly 12 metric entries', async () => {
+  it('contains exactly 14 metric entries', async () => {
     const entries = await metricsRegistry.getMetricsAsJSON();
-    expect(entries).toHaveLength(12);
+    expect(entries).toHaveLength(14);
   });
 
   // ──────────────────────────────────────────────────────────────────
@@ -54,6 +56,8 @@ describe('metricsRegistry', () => {
     'agentidp_rate_limit_hits_total',
     'agentidp_db_pool_active_connections',
     'agentidp_db_pool_waiting_requests',
+    'agentidp_tenant_api_calls_total',
+    'agentidp_billing_limit_rejections_total',
   ])('registers metric "%s"', async (metricName) => {
     const entries = await metricsRegistry.getMetricsAsJSON();
     const names = entries.map((e) => e.name);
@@ -200,4 +204,33 @@ describe('metricsRegistry', () => {
       expect(() => dbPoolWaitingRequests.set(2)).not.toThrow();
     });
   });
+
+  describe('tenantApiCallsTotal', () => {
+    it('has name agentidp_tenant_api_calls_total', () => {
+      const metric = tenantApiCallsTotal as unknown as { name: string };
+      expect(metric.name).toBe('agentidp_tenant_api_calls_total');
+    });
+
+    it('increments with tenant_id label without throwing', () => {
+      expect(() =>
+        tenantApiCallsTotal.inc({ tenant_id: 'org-test-001' }),
+      ).not.toThrow();
+    });
+  });
+
+  describe('billingLimitRejectionsTotal', () => {
+    it('has name agentidp_billing_limit_rejections_total', () => {
+      const metric = billingLimitRejectionsTotal as unknown as { name: string };
+      expect(metric.name).toBe('agentidp_billing_limit_rejections_total');
+    });
+
+    it('increments with tenant_id and limit_type labels without throwing', () => {
+      expect(() =>
+        billingLimitRejectionsTotal.inc({ tenant_id: 'org-test-001', limit_type: 'agent_limit' }),
+      ).not.toThrow();
+      expect(() =>
+        billingLimitRejectionsTotal.inc({ tenant_id: 'org-test-002', limit_type: 'api_limit' }),
+      ).not.toThrow();
+    });
+  });
 });