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>

lib/accounting/quickbooks.ts

@@ -1,4 +1,4 @@
-import type { AccountingProvider, OAuthTokens, IncomeEntry, ExpenseEntry } from "./types"
+import type { AccountingProvider, OAuthTokens } from "./types"
 import { redirectUri } from "./types"
 
 // QuickBooks Online. Docs: https://developer.intuit.com/app/developer/qbo/docs/develop

lib/accounting/xero.ts

@@ -1,4 +1,4 @@
-import type { AccountingProvider, OAuthTokens, IncomeEntry, ExpenseEntry } from "./types"
+import type { AccountingProvider, OAuthTokens } from "./types"
 import { redirectUri } from "./types"
 
 // Xero. Docs: https://developer.xero.com/documentation/guides/oauth2/