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>

src/metrics/registry.ts

@@ -159,3 +159,16 @@ export const tenantApiCallsTotal = new Counter({
   labelNames: ['tenant_id'] as const,
   registers: [metricsRegistry],
 });
+
+/**
+ * Total number of requests rejected due to free tier billing limits.
+ * Labels: tenant_id, limit_type ('agent_limit' | 'api_limit')
+ *
+ * WS6 — Billing & Usage Metering.
+ */
+export const billingLimitRejectionsTotal = new Counter({
+  name: 'agentidp_billing_limit_rejections_total',
+  help: 'Total number of requests rejected due to free tier billing limits.',
+  labelNames: ['tenant_id', 'limit_type'] as const,
+  registers: [metricsRegistry],
+});