fix: add description to VideoModel type and GraphQL schema

Requesting description on the article author caused a GraphQL error
which the page.server.ts caught as a 404.

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

packages/backend/src/graphql/types/index.ts

@@ -86,6 +86,7 @@ export const VideoModelType = builder.objectRef<VideoModel>("VideoModel").implem
     artist_name: t.exposeString("artist_name", { nullable: true }),
     slug: t.exposeString("slug", { nullable: true }),
     avatar: t.exposeString("avatar", { nullable: true }),
+    description: t.exposeString("description", { nullable: true }),
   }),
 });