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/(marketing)/layout.tsx

@@ -1,6 +1,6 @@
 import { Navbar } from "@/components/marketing/navbar"
 import { Footer } from "@/components/marketing/footer"
-import { StructuredData } from "@/components/marketing/structured-data"
+import { SiteStructuredData } from "@/components/marketing/structured-data"
 import { getSession, isAdminUser } from "@/lib/session"
 import { getMaintenanceMode } from "@/lib/settings"
 import { MaintenanceScreen } from "@/components/shared/maintenance-screen"
@@ -17,7 +17,7 @@ export default async function MarketingLayout({ children }: { children: React.Re
 
   return (
     <div className="min-h-screen bg-[#09090b] text-white">
-      <StructuredData />
+      <SiteStructuredData />
       <Navbar />
       {children}
       <Footer />