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/App.tsx

@@ -9,6 +9,7 @@ import AgentDetail from '@/pages/AgentDetail';
 import Credentials from '@/pages/Credentials';
 import AuditLog from '@/pages/AuditLog';
 import Health from '@/pages/Health';
+import { UsagePanel } from '@/components/UsagePanel';
 
 /** Top-level router — defines all application routes. */
 export default function App(): React.JSX.Element {
@@ -23,6 +24,7 @@ export default function App(): React.JSX.Element {
             <Route path="/dashboard/agents/:agentId/credentials" element={<Credentials />} />
             <Route path="/dashboard/audit" element={<AuditLog />} />
             <Route path="/dashboard/health" element={<Health />} />
+            <Route path="/dashboard/usage" element={<UsagePanel />} />
           </Route>
         </Route>
         <Route path="/dashboard" element={<Navigate to="/dashboard/agents" replace />} />