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>

dashboard/src/components/layout/AppShell.tsx

@@ -12,6 +12,7 @@ const NAV_ITEMS: NavItem[] = [
   { to: '/dashboard/agents',  label: 'Agents' },
   { to: '/dashboard/audit',   label: 'Audit Log' },
   { to: '/dashboard/health',  label: 'Health' },
+  { to: '/dashboard/usage',   label: 'Usage' },
 ];
 
 /**