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/016_add_auth_security_columns.js

@@ -0,0 +1,25 @@
+exports.up = function(knex) {
+  return knex.schema.table('admin_users', table => {
+    // Add password change tracking
+    table.timestamp('password_changed_at').nullable();
+    
+    // Add last login IP for security monitoring
+    table.string('last_login_ip', 45).nullable();
+    
+    // Add account security flags
+    table.boolean('two_factor_enabled').defaultTo(false);
+    table.string('two_factor_secret').nullable();
+    
+    // Add index for performance
+    table.index('password_changed_at');
+  });
+};
+
+exports.down = function(knex) {
+  return knex.schema.table('admin_users', table => {
+    table.dropColumn('password_changed_at');
+    table.dropColumn('last_login_ip');
+    table.dropColumn('two_factor_enabled');
+    table.dropColumn('two_factor_secret');
+  });
+};