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/calendar/[token]/route.ts

@@ -115,7 +115,12 @@ export async function GET(_req: Request, { params }: { params: Promise<{ token:
     headers: {
       "Content-Type": "text/calendar; charset=utf-8",
       "Content-Disposition": 'inline; filename="property-management-network.ics"',
-      "Cache-Control": "public, max-age=3600",
+      // PRIVATE, never shared-cacheable: the only credential is the token in the
+      // URL, and the body carries tenant names, rent amounts, property addresses
+      // and lease dates. A `public` cache directive would let any intermediary
+      // or CDN retain that PII.
+      "Cache-Control": "private, max-age=3600",
+      "X-Robots-Tag": "noindex, nofollow",
     },
   })
 }