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/api/ai/rent-receipt/route.ts

@@ -38,7 +38,9 @@ export async function POST(request: Request) {
   }
 
   // Pass only the whitelisted, validated fields to the model.
-  const { payment_id, ...receiptFields } = parsed.data
+  // payment_id identifies the row but must not reach the model — destructured
+  // out deliberately, hence the leading underscore.
+  const { payment_id: _payment_id, ...receiptFields } = parsed.data
 
   const text = await aiComplete({
     messages: [