fix: PostgreSQL insert compatibility issues

- Fix event creation failing with 'not iterable' error
- Add .returning('id') to insert queries for PostgreSQL
- Handle both PostgreSQL (returns objects) and SQLite (returns IDs)
- Add missing fields to email_queue insert

This fixes the 500 error when creating new events in production.

backend/src/routes/adminCategories.js

@@ -77,12 +77,14 @@ router.post('/', adminAuth, [
     }
     
     // Create category
-    const [categoryId] = await db('photo_categories').insert({
+    const insertResult = await db('photo_categories').insert({
       name,
       slug: categorySlug,
       is_global,
       event_id: is_global ? null : event_id
-    });
+    }).returning('id');
+    
+    const categoryId = insertResult[0]?.id || insertResult[0];
     
     const category = await db('photo_categories').where('id', categoryId).first();