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/billing/checkout/route.ts

@@ -0,0 +1,41 @@
+import { auth } from "@/app/(auth)/auth";
+import {
+  createCheckoutSession,
+  isBillingConfigured,
+} from "@/lib/billing/gateway";
+import { ChatbotError } from "@/lib/errors";
+
+export async function POST(request: Request) {
+  const session = await auth();
+  if (!session?.user) {
+    return new ChatbotError("unauthorized:chat").toResponse();
+  }
+
+  if (!isBillingConfigured()) {
+    return Response.json(
+      {
+        error: "billing_not_configured",
+        message:
+          "Set EGBE_PAYMENT_GATEWAY_URL, EGBE_GATEWAYS_TOKEN, and PRO_STRIPE_PRICE_ID to enable upgrades.",
+      },
+      { status: 501 }
+    );
+  }
+
+  const appSlug = process.env.APP_SLUG ?? "chatbot";
+  const baseUrl =
+    process.env.NEXT_PUBLIC_BASE_URL ?? new URL(request.url).origin;
+
+  try {
+    const result = await createCheckoutSession({
+      userId: session.user.id,
+      customerEmail: session.user.email ?? undefined,
+      appSlug,
+      baseUrl,
+    });
+    return Response.json({ url: result.url, id: result.id });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : "checkout failed";
+    return Response.json({ error: "checkout_failed", message }, { status: 502 });
+  }
+}