fix: add adminGetVideo/adminGetArticle queries to fix 404 on edit pages

The edit page loaders were calling adminListVideos/adminListArticles with the
old pre-pagination signatures and filtering by ID client-side, which broke
after pagination limited results to 50. Now fetches the single item by ID
directly via new adminGetVideo and adminGetArticle backend queries.

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

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

@@ -1,16 +1,17 @@
-import { adminListArticles, adminListUsers } from "$lib/services";
+import { adminGetArticle, adminListUsers } from "$lib/services";
 import { error } from "@sveltejs/kit";
 
 export async function load({ params, fetch, cookies }) {
   const token = cookies.get("session_token") || "";
-  const [articles, modelsResult] = await Promise.all([
-    adminListArticles(fetch, token).catch(() => []),
+  const [article, modelsResult] = await Promise.all([
+    adminGetArticle(params.id, fetch, token).catch(() => null),
     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, authors: modelsResult.items };
 }