feat: dynamic magazine category filter from published articles

Add articleCategories GraphQL query returning distinct categories from
published articles; magazine page fetches them at load time and renders
only categories that actually have content.

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

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

@@ -1,7 +1,7 @@
 import { builder } from "../builder";
 import { ArticleType, ArticleListType, AdminArticleListType } from "../types/index";
 import { articles, users } from "../../db/schema/index";
-import { eq, and, lte, desc, asc, ilike, or, count, arrayContains, type SQL } from "drizzle-orm";
+import { eq, and, lte, desc, asc, ilike, or, count, arrayContains, isNotNull, type SQL } from "drizzle-orm";
 import { requireAdmin } from "../../lib/acl";
 import type { DB } from "../../db/connection";
 
@@ -74,6 +74,20 @@ builder.queryField("articles", (t) =>
   }),
 );
 
+builder.queryField("articleCategories", (t) =>
+  t.field({
+    type: ["String"],
+    resolve: async (_root, _args, ctx) => {
+      const rows = await ctx.db
+        .selectDistinct({ category: articles.category })
+        .from(articles)
+        .where(and(lte(articles.publish_date, new Date()), isNotNull(articles.category)))
+        .orderBy(asc(articles.category));
+      return rows.map((r) => r.category!);
+    },
+  }),
+);
+
 builder.queryField("article", (t) =>
   t.field({
     type: ArticleType,