Reorganize migrations for new vs existing deployments

- Created core/ directory for essential migrations that always run
- Created legacy/ directory for migrations only needed when upgrading
- New deployments will only run core migrations for a clean database
- Existing deployments will run all migrations in proper sequence
- Fixed duplicate migration numbers (014 and 027)
- Updated migration runners to handle new directory structure
- Added README explaining the migration organization

This change optimizes deployment for new users who will get a clean
schema without running unnecessary upgrade migrations.

backend/migrations/legacy/027_add_rate_limit_settings_duplicate.js

@@ -0,0 +1,62 @@
+exports.up = async function(knex) {
+  // Add rate limit settings to app_settings
+  const rateLimitSettings = [
+    {
+      setting_key: 'rate_limit_enabled',
+      setting_value: JSON.stringify(true),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_window_minutes',
+      setting_value: JSON.stringify(15),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_max_requests',
+      setting_value: JSON.stringify(1000),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_auth_max_requests',
+      setting_value: JSON.stringify(5),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_skip_authenticated',
+      setting_value: JSON.stringify(true),
+      setting_type: 'security'
+    },
+    {
+      setting_key: 'rate_limit_public_endpoints_only',
+      setting_value: JSON.stringify(false),
+      setting_type: 'security'
+    }
+  ];
+
+  // Insert settings if they don't exist
+  for (const setting of rateLimitSettings) {
+    const exists = await knex('app_settings')
+      .where('setting_key', setting.setting_key)
+      .first();
+    
+    if (!exists) {
+      await knex('app_settings').insert({
+        ...setting
+      });
+    }
+  }
+};
+
+exports.down = async function(knex) {
+  // Remove rate limit settings
+  await knex('app_settings')
+    .whereIn('setting_key', [
+      'rate_limit_enabled',
+      'rate_limit_window_minutes',
+      'rate_limit_max_requests',
+      'rate_limit_auth_max_requests',
+      'rate_limit_skip_authenticated',
+      'rate_limit_public_endpoints_only'
+    ])
+    .del();
+};