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>

.env.production.example

@@ -135,5 +135,14 @@ CRON_SECRET=replace-with-a-random-string
 # Create a widget at https://dash.cloudflare.com/?to=/:account/turnstile
 # NEXT_PUBLIC_TURNSTILE_SITE_KEY is inlined at build time — also set it as a
 # Build Variable in Coolify. Leave both blank to disable the captcha.
+# NOTE: in production a blank TURNSTILE_SECRET_KEY now FAILS CLOSED — auth
+# forms are rejected rather than silently losing bot protection.
 NEXT_PUBLIC_TURNSTILE_SITE_KEY=
 TURNSTILE_SECRET_KEY=
+
+# === SECRETS AT REST ===
+# AES-256-GCM key for the OAuth tokens stored for QuickBooks / Xero / DocuSign /
+# Dropbox Sign. If unset, the key is derived from BETTER_AUTH_SECRET — which
+# means rotating BETTER_AUTH_SECRET would make every stored token undecryptable.
+# Set a dedicated value in production so the two can rotate independently.
+ACCOUNTING_ENCRYPTION_KEY=replace-with-a-random-string