Fix email queue column error in event creation

- Remove created_at field from email_queue insert (table uses scheduled_at)
- Let scheduled_at use its default value from database schema

This fixes the 500 error that occurred when creating events due to
trying to insert a non-existent column.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/src/routes/adminEvents.js

@@ -98,8 +98,8 @@ router.post('/', adminAuth, [
         share_link: shareLink,
         password,
         expires_at: expires_at.toISOString()
-      }),
-      created_at: new Date()
+      })
+      // scheduled_at will use default value
     });
     
     res.json({