Add Umami analytics, subscription tiers, tool-call metering

- Analytics: <Analytics /> in root layout loads umami when NEXT_PUBLIC_UMAMI_* env are set
- Subscription: free/pro tiers in Postgres, daily quotas (10msg/5tool free, 1000/500 pro)
- Chat route: checkAndConsume(chat_message) gate before streamText, 429 when exhausted
- Tool metering: getWeather wrapped with withLimit decorator that deducts tool_call quota
- Pricing page at /pricing with upgrade button
- /api/billing/checkout posts to EGBE payment gateway, /api/billing/webhook verifies HMAC and upgrades user on checkout.session.completed
- UsageBadge in chat header polls /api/usage every 15s
- Dropped now-unused entitlements.ts

proxy.ts

@@ -13,6 +13,10 @@ export async function proxy(request: NextRequest) {
     return NextResponse.next();
   }
 
+  if (pathname.startsWith("/api/billing/webhook")) {
+    return NextResponse.next();
+  }
+
   const token = await getToken({
     req: request,
     secret: process.env.AUTH_SECRET,