feat(cms): LegalPages collection + migration (MITHOME-88)

Slug-based collection mirroring LegalPageContent (proto/src/content/
types.ts) for the two legal pages (adatvedelem, hasznalati-feltetelek).

- src/collections/LegalPages.ts: slug (unique), title, lastUpdated,
  sections[] (id/title/content). `content` stays a plain textarea, not
  lexical richText — the current frontend
  (src/app/(frontend)/adatvedelem/page.tsx) renders it through a
  hand-rolled "•"/"**bold**" regex converter, not a real Markdown/
  richText parser, matching the same bootstrap-scope call made for
  Contact.gdpr.label in MITHOME-87.
- Registered in payload.config.ts.
- migrate-content-to-payload.ts: added an idempotent upsertLegalPage
  helper (find-by-slug, then update or create — Collections don't have
  Globals' fixed-slug updateGlobal) and seeded both legal pages from
  their existing JSON.

Verified: migration run twice against the real dev MongoDB produced
exactly 2 documents (no duplicates) — confirmed via mongosh. Real
browser: logged into /admin, Legal Pages list shows both entries with
correct titles/slugs, opened the adatvedelem document and the slug/
title/body fields all show the migrated content correctly. build/lint/
tsc/test (58 passed) all clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

proto/src/payload.config.ts

@@ -7,7 +7,8 @@
  *
  * MITHOME-87: Home/About/Services/Contact/Common Globals hozzáadva — ezek
  * tükrözik a proto/src/content/pages/*.json + common.json struktúráját.
- * A LegalPages/Partners/Media collectionök a MITHOME-88..89-ben kerülnek be.
+ * MITHOME-88: LegalPages collection hozzáadva (adatvedelem, hasznalati-
+ * feltetelek). A Partners/Media collectionök a MITHOME-89-ben kerülnek be.
  */
 import path from 'path'
 import { fileURLToPath } from 'url'
@@ -17,6 +18,7 @@ import { lexicalEditor } from '@payloadcms/richtext-lexical'
 import sharp from 'sharp'
 
 import { Users } from './collections/Users'
+import { LegalPages } from './collections/LegalPages'
 import { Home } from './globals/Home'
 import { About } from './globals/About'
 import { Services } from './globals/Services'
@@ -35,7 +37,7 @@ export default buildConfig({
 
   editor: lexicalEditor(),
 
-  collections: [Users],
+  collections: [Users, LegalPages],
 
   globals: [Home, About, Services, Contact, Common],