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/backend/src/graphql/resolvers/articles.ts

@@ -96,6 +96,26 @@ builder.queryField("article", (t) =>
   }),
 );
 
+builder.queryField("adminGetArticle", (t) =>
+  t.field({
+    type: ArticleType,
+    nullable: true,
+    args: {
+      id: t.arg.string({ required: true }),
+    },
+    resolve: async (_root, args, ctx) => {
+      requireAdmin(ctx);
+      const article = await ctx.db
+        .select()
+        .from(articles)
+        .where(eq(articles.id, args.id))
+        .limit(1);
+      if (!article[0]) return null;
+      return enrichArticle(ctx.db, article[0]);
+    },
+  }),
+);
+
 // ─── Admin queries & mutations ────────────────────────────────────────────────
 
 builder.queryField("adminListArticles", (t) =>

packages/backend/src/graphql/resolvers/videos.ts

@@ -188,6 +188,22 @@ builder.queryField("video", (t) =>
   }),
 );
 
+builder.queryField("adminGetVideo", (t) =>
+  t.field({
+    type: VideoType,
+    nullable: true,
+    args: {
+      id: t.arg.string({ required: true }),
+    },
+    resolve: async (_root, args, ctx) => {
+      requireAdmin(ctx);
+      const video = await ctx.db.select().from(videos).where(eq(videos.id, args.id)).limit(1);
+      if (!video[0]) return null;
+      return enrichVideo(ctx.db, video[0]);
+    },
+  }),
+);
+
 builder.queryField("videoLikeStatus", (t) =>
   t.field({
     type: VideoLikeStatusType,