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

components/analytics.tsx

@@ -0,0 +1,20 @@
+import Script from "next/script";
+
+export function Analytics() {
+  const websiteId = process.env.NEXT_PUBLIC_UMAMI_WEBSITE_ID;
+  const scriptUrl = process.env.NEXT_PUBLIC_UMAMI_SCRIPT_URL;
+
+  if (!(websiteId && scriptUrl)) {
+    return null;
+  }
+
+  return (
+    <Script
+      async
+      data-website-id={websiteId}
+      defer
+      src={scriptUrl}
+      strategy="afterInteractive"
+    />
+  );
+}