feat(categories): CRUD endpoints + is_active for archiving

Endpoints under /api/admin/categories:
  GET    — list all with metrics (article_count, topic_count, autogen status)
  POST   — create + auto-create autogen_settings row (defaults: enabled, 1/day, 12:00 MSK)
  PATCH  — update name/desc/icon/color/sort_order/is_active (slug is immutable FK)
  DELETE — soft (is_active=false) by default; ?force=true tries hard delete
           but refuses if any articles/topics still reference the slug

Migration: ALTER TABLE categories ADD COLUMN IF NOT EXISTS is_active BOOLEAN DEFAULT true
Public GET /api/categories now filters WHERE COALESCE(is_active,true)=true.

src/config/db.js

@@ -221,6 +221,10 @@ const migrate = async () => {
     CREATE INDEX IF NOT EXISTS idx_zero_notes_published ON zero_notes(published_at DESC) WHERE status='published';
   `);
 
+
+  // safe column alters (existing tables on prod may lack newer columns)
+  await query(`ALTER TABLE categories ADD COLUMN IF NOT EXISTS is_active BOOLEAN DEFAULT true`);
+
   console.log('[DB] Migrations applied');
 };