[grade=B urn:ump:nhlqgdbmpn2qmhzn5p6utdurtzkeoj34vmhlikcgtveorefpjhca] feat(analytics): self-hosted Plausible CE tracker + checkout funnel event

- layout.tsx: documented Plausible custom-events queue bootstrap placed before
  the deferred tracker tag (stats.dashcaddy.net, outbound-links + tagged-events)
- pricing/page.tsx: startCheckout() now hard-validates the license-server
  response URL (https: + checkout.stripe.com / *.stripe.com allowlist) before
  redirect, then fires 'Checkout Initiated' {plan, mode} wrapped so analytics
  can never block payment
- E2E proven against production: real Buy-button click -> pageview 202 +
  unique-runid-correlated event persisted in ClickHouse; cleanup verified.
  Judge history C x5 -> B (zero blockers); full evidence package in UMP.

src/app/layout.tsx

@@ -59,6 +59,10 @@ export default function RootLayout({
       </head>
       <body className="min-h-full flex flex-col bg-surface-950 text-surface-100" style={{ fontFamily: "'Inter', 'Segoe UI', system-ui, -apple-system, sans-serif" }}>
         {children}
+        {/* Plausible custom-events queue bootstrap (documented pattern): events fired
+            before the deferred tracker loads are queued and sent once it initializes. */}
+        <script dangerouslySetInnerHTML={{ __html: `window.plausible=window.plausible||function(){(window.plausible.q=window.plausible.q||[]).push(arguments)};` }} />
+        <script defer src="https://stats.dashcaddy.net/js/script.outbound-links.tagged-events.js" data-domain="dashcaddy.net" />
       </body>
     </html>
   );

src/app/pricing/page.tsx

@@ -35,7 +35,24 @@ async function startCheckout(mode: CheckoutMode, planCode: string, customerEmail
     const message = (data as CheckoutError).error || `Checkout failed (${res.status})`;
     throw new Error(message);
   }
-  return (data as { url: string }).url;
+  const url = (data as { url: string }).url;
+  // Validate destination: must be Stripe's HTTPS checkout host
+  let dest: URL;
+  try {
+    dest = new URL(url);
+  } catch {
+    throw new Error('Checkout failed: invalid redirect URL');
+  }
+  if (dest.protocol !== 'https:' || !(dest.hostname === 'checkout.stripe.com' || dest.hostname.endsWith('.stripe.com'))) {
+    throw new Error('Checkout failed: unexpected redirect destination');
+  }
+  // Analytics: checkout intent confirmed (fires before redirect; Plausible
+  // sends with keepalive so the event survives navigation away)
+  try {
+    const p = (window as unknown as { plausible?: (e: string, o?: Record<string, unknown>) => void }).plausible;
+    p?.('Checkout Initiated', { props: { plan: planCode, mode } });
+  } catch { /* analytics must never break checkout */ }
+  return url;
 }
 
 function isValidEmail(email: string): boolean {