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/src/routes/admin/+layout.svelte

@@ -0,0 +1,50 @@
+<script lang="ts">
+  import { page } from "$app/state";
+
+  const { children } = $props();
+
+  const navLinks = [
+    { name: "Users", href: "/admin/users", icon: "icon-[ri--team-line]" },
+    { name: "Videos", href: "/admin/videos", icon: "icon-[ri--film-line]" },
+    { name: "Articles", href: "/admin/articles", icon: "icon-[ri--article-line]" },
+  ];
+
+  function isActive(href: string) {
+    return page.url.pathname.startsWith(href);
+  }
+</script>
+
+<div class="flex min-h-screen bg-background">
+  <!-- Sidebar -->
+  <aside
+    class="w-56 shrink-0 border-r border-border/40 bg-card/60 backdrop-blur-sm flex flex-col"
+  >
+    <div class="px-4 py-5 border-b border-border/40">
+      <a href="/" class="text-xs text-muted-foreground hover:text-foreground transition-colors">
+        ← Back to site
+      </a>
+      <h1 class="mt-2 text-base font-bold text-foreground">Admin</h1>
+    </div>
+
+    <nav class="flex-1 p-3 space-y-1">
+      {#each navLinks as link (link.href)}
+        <a
+          href={link.href}
+          class={`flex items-center gap-3 rounded-lg px-3 py-2 text-sm font-medium transition-colors ${
+            isActive(link.href)
+              ? "bg-primary/10 text-primary"
+              : "text-muted-foreground hover:text-foreground hover:bg-muted/50"
+          }`}
+        >
+          <span class={`${link.icon} h-4 w-4`}></span>
+          {link.name}
+        </a>
+      {/each}
+    </nav>
+  </aside>
+
+  <!-- Main content -->
+  <main class="flex-1 overflow-auto">
+    {@render children()}
+  </main>
+</div>