feat: role-based ACL + admin management UI

Backend:
- Add acl.ts with requireAuth/requireRole/requireOwnerOrAdmin helpers
- Gate premium videos from unauthenticated users in videos query/resolver
- Fix updateVideoPlay to verify ownership before updating
- Add admin mutations: adminListUsers, adminUpdateUser, adminDeleteUser
- Add admin mutations: createVideo, updateVideo, deleteVideo, setVideoModels, adminListVideos
- Add admin mutations: createArticle, updateArticle, deleteArticle, adminListArticles
- Add deleteComment mutation (owner or admin only)
- Add AdminUserListType to GraphQL types
- Fix featured filter on articles query

Frontend:
- Install marked for markdown rendering
- Add /admin/* section with sidebar layout and admin-only guard
- Admin users page: paginated table with search, role filter, inline role change, delete
- Admin videos pages: list, create form, edit form with file upload and model assignment
- Admin articles pages: list, create form, edit form with split-pane markdown editor
- Add admin nav link in header (desktop + mobile) for admin users
- Render article content through marked in magazine detail page
- Add all admin GraphQL service functions to services.ts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/package.json

@@ -47,6 +47,7 @@
     "graphql": "^16.11.0",
     "graphql-request": "^7.1.2",
     "javascript-time-ago": "^2.6.4",
+    "marked": "^17.0.4",
     "media-chrome": "^4.18.0",
     "svelte-i18n": "^4.0.1"
   }