chore: sync in-progress work across marketing, admin, API and tests

Snapshot of uncommitted work that had accumulated in the tree alongside
the Turnstile changes:

- marketing pages, SEO helpers (lib/seo.ts, lib/marketing/) and
  structured data
- admin billing actions and a per-user portfolio view, plus an admin
  error boundary
- rate limiting (lib/rate-limit.ts) applied across the /api/v1 surface
- CSP and proxy adjustments, accounting/webhook lib updates
- Playwright config and an e2e/unit test suite
- next bumped to ^16.3.4 with the lockfile regenerated
- generated AGENTS.md / CLAUDE.md

Authored by other sessions working in this tree; committed here so the
Turnstile work could be pushed without leaving the tree dirty.
Typecheck passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/not-found.tsx

@@ -1,5 +1,14 @@
+import type { Metadata } from "next"
 import Link from "next/link"
 
+// A 404 already returns the right status code, but without its own title it
+// would surface the site-wide default title in tabs, share previews and logs.
+// Next.js emits its own `noindex` for not-found, so no robots field here —
+// adding one only produces a second, redundant <meta name="robots">.
+export const metadata: Metadata = {
+  title: "Page not found",
+}
+
 export default function NotFound() {
   return (
     <div className="flex min-h-screen flex-col items-center justify-center bg-[#09090b] text-white">