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/crypto.ts

@@ -3,9 +3,27 @@ import crypto from "crypto"
 // AES-256-GCM encryption for secrets at rest (OAuth tokens). The key is derived
 // from ACCOUNTING_ENCRYPTION_KEY, falling back to BETTER_AUTH_SECRET, via SHA-256
 // so no additional configuration is required.
+let warnedAboutFallbackKey = false
+
 function getKey(): Buffer {
-  const secret = process.env.ACCOUNTING_ENCRYPTION_KEY || process.env.BETTER_AUTH_SECRET
+  const dedicated = process.env.ACCOUNTING_ENCRYPTION_KEY
+  const secret = dedicated || process.env.BETTER_AUTH_SECRET
   if (!secret) throw new Error("No encryption key configured (BETTER_AUTH_SECRET)")
+
+  // Riding on BETTER_AUTH_SECRET works, but couples two independent rotation
+  // schedules: rotating the auth secret would silently make every stored OAuth
+  // token undecryptable, with no migration path and no error until a user's
+  // next accounting sync fails. Warn once so this is caught before that
+  // happens rather than after.
+  if (!dedicated && !warnedAboutFallbackKey) {
+    warnedAboutFallbackKey = true
+    console.warn(
+      "[crypto] ACCOUNTING_ENCRYPTION_KEY is not set — deriving the at-rest key " +
+        "from BETTER_AUTH_SECRET. Rotating BETTER_AUTH_SECRET will make all " +
+        "stored OAuth tokens undecryptable. Set a dedicated key in production."
+    )
+  }
+
   return crypto.createHash("sha256").update(secret).digest()
 }