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

app/(chat)/api/usage/route.ts

@@ -0,0 +1,16 @@
+import { auth } from "@/app/(auth)/auth";
+import { ChatbotError } from "@/lib/errors";
+import { getQuotaSummary } from "@/lib/subscription/service";
+
+export async function GET() {
+  const session = await auth();
+  if (!session?.user) {
+    return new ChatbotError("unauthorized:chat").toResponse();
+  }
+  const summary = await getQuotaSummary(session.user.id);
+  return Response.json(summary, {
+    headers: {
+      "Cache-Control": "no-store",
+    },
+  });
+}