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>

package.json

@@ -38,6 +38,7 @@
     "prom-client": "^15.1.3",
     "rate-limiter-flexible": "^5.0.5",
     "redis": "^4.6.13",
+    "stripe": "^21.0.1",
     "ulid": "^3.0.2",
     "uuid": "^9.0.1",
     "web-did-resolver": "^2.0.32"