fix: remove updated_at from app_settings inserts in multiple migrations

The app_settings table in production doesn't have created_at/updated_at columns.
Fixed inconsistent usage across migrations:
- Migration 014: removed updated_at: new Date()
- Migration 027: removed updated_at: knex.fn.now()
- Migration 033: removed updated_at: new Date()

This ensures all migrations are consistent and won't fail in production.

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

Co-Authored-By: Claude <[email protected]>

backend/migrations/027_add_rate_limit_settings.js

@@ -41,8 +41,7 @@ exports.up = async function(knex) {
     
     if (!exists) {
       await knex('app_settings').insert({
-        ...setting,
-        updated_at: knex.fn.now()
+        ...setting
       });
     }
   }