forum: drop Recent Threads widget, surface last post inline on category cards

Per Ryan — category drill-down already shows newest-first ordering so
the standalone Recent Threads list was redundant. Each category card now
shows "Last post: <title> · <relative time>" below the description so
glanceable activity stays on the landing page. Categories API joins one
latest thread per category in a single round-trip (in_ + limit + group
in memory) instead of N+1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/api/forum/categories/route.ts

@@ -4,13 +4,42 @@ import { createClient } from '@/lib/supabase/server';
 export async function GET() {
   try {
     const supabase = await createClient();
-    const { data, error } = await supabase
+    const { data: cats, error } = await supabase
       .from('forum_categories')
       .select('*')
       .order('display_order', { ascending: true });
 
     if (error) throw error;
-    return NextResponse.json({ categories: data });
+
+    // Latest thread per category — joined client-side so we keep one
+    // round-trip per category instead of N+1. Each cat ends up with
+    // last_thread_title + last_activity_at attached.
+    const catIds = (cats || []).map((c: any) => c.id);
+    let lastByCat: Record<string, { title: string; at: string }> = {};
+    if (catIds.length) {
+      const { data: latest } = await supabase
+        .from('forum_threads')
+        .select('id, category_id, title, last_reply_at, created_at')
+        .in('category_id', catIds)
+        .order('last_reply_at', { ascending: false, nullsFirst: false })
+        .limit(2000);
+      for (const t of latest || []) {
+        const k = t.category_id as string;
+        if (!lastByCat[k]) {
+          lastByCat[k] = {
+            title: t.title,
+            at:    t.last_reply_at || t.created_at,
+          };
+        }
+      }
+    }
+    const enriched = (cats || []).map((c: any) => ({
+      ...c,
+      last_thread_title: lastByCat[c.id]?.title || null,
+      last_activity_at:  lastByCat[c.id]?.at    || null,
+    }));
+
+    return NextResponse.json({ categories: enriched });
   } catch (err: any) {
     return NextResponse.json({ error: err.message }, { status: 500 });
   }