refactor: align article author with VideoModel, streamline selects, fix flyout inert

- Remove ArticleAuthor type; article.author now reuses VideoModel (id, artist_name, slug, avatar)
- updateArticle accepts authorId; author selectable in admin article edit page
- Article edit: single Select with bind:value + $derived selectedAuthor display
- Video edit: replace pill toggles with Select type="multiple" bind:value for models
- Video table: replace inline badge spans with Badge component
- Magazine: display artist_name throughout, author bio links to model profile
- Fix flyout aria-hidden warning: replace with inert attribute

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

packages/frontend/src/routes/admin/articles/[id]/+page.server.ts

@@ -1,10 +1,13 @@
-import { adminListArticles } from "$lib/services";
+import { adminListArticles, adminListUsers } from "$lib/services";
 import { error } from "@sveltejs/kit";
 
 export async function load({ params, fetch, cookies }) {
   const token = cookies.get("session_token") || "";
-  const articles = await adminListArticles(fetch, token).catch(() => []);
+  const [articles, modelsResult] = await Promise.all([
+    adminListArticles(fetch, token).catch(() => []),
+    adminListUsers({ role: "model", limit: 200 }, fetch, token).catch(() => ({ items: [], total: 0 })),
+  ]);
   const article = articles.find((a) => a.id === params.id);
   if (!article) throw error(404, "Article not found");
-  return { article };
+  return { article, authors: modelsResult.items };
 }