Add pages API: model, controller, routes, preview (steps 4/6/7/8)

Backend for the CMS page builder, all under the existing /api/v1:

- pages.model: authoritative save gate — validates blocks against the
  registry and sanitizes them on every create/update; maps rows to/from the
  grouped API shape (metadata / settings); slug validated + reserved-checked
  at create and immutable after; `protected` can be set true via PATCH but
  only cleared via the unprotect path; published_at stamped on first publish.
- sanitizeBlocks: post-validation normalizer (applies each block's sanitize,
  stamps version, defaults visible, recurses container slots).
- reservedSlugs: guards page slugs from shadowing named routes/API namespaces.
- Admin routes (staff-gated): GET/POST /pages, GET/PATCH/DELETE /pages/:id,
  POST /pages/:id/unprotect (password step-up, verified against the caller's
  own hash, never logged), POST /pages/:id/preview (1h token). Audit-logs
  create/publish/unpublish/protect/unprotect/delete.
- Public routes: GET /public/pages/:slug (published; staff see drafts; site-
  mode gated) and GET /public/pages/:id/preview/:token (ungated, token is the
  access control). Preview token primitives added to auth/token.js.
- Swagger annotations for all new endpoints.

Verified end-to-end: model integration test against the dev DB (sanitize,
invalid-block rejection, slug immutability, protected/unprotect, dup/reserved
slug, published_at) + authenticated HTTP smoke (201 create, 400 invalid
blocks, publish, public slug fetch, preview mint+fetch, 403 delete-protected,
401 wrong-password unprotect).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/auth/token.js

@@ -66,6 +66,20 @@ function verifyTotpChallenge(token) {
   return decoded
 }
 
+// Short-lived, unguessable link token for previewing a (possibly unpublished)
+// CMS page. Carries purpose:'page_preview' + the page id and nothing else; it is
+// NOT a session (session validation rejects it) and only grants read of that one
+// page's current block state. Default 1h expiry per the page-builder spec.
+function signPagePreview(pageId, { expiresIn = '1h' } = {}) {
+  return jwt.sign({ pageId, purpose: 'page_preview' }, JWT_SECRET, { expiresIn })
+}
+
+function verifyPagePreview(token) {
+  const decoded = verifyToken(token)
+  if (!decoded || decoded.purpose !== 'page_preview') return null
+  return decoded
+}
+
 // Rough max-age (ms) for the cookie, parsed from JWT_EXPIRES_IN (e.g. 1d, 12h, 30m).
 function cookieMaxAge() {
   const m = /^(\d+)([dhms])$/.exec(String(JWT_EXPIRES_IN).trim())
@@ -122,6 +136,8 @@ module.exports = {
   verifyToken,
   signTotpChallenge,
   verifyTotpChallenge,
+  signPagePreview,
+  verifyPagePreview,
   cookieMaxAge,
   cookieSecure,
   cookieOptions,