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/015_add_login_attempts_table.js

@@ -0,0 +1,19 @@
+exports.up = function(knex) {
+  return knex.schema.createTable('login_attempts', table => {
+    table.increments('id').primary();
+    table.string('identifier').notNullable(); // username or email
+    table.string('ip_address', 45).notNullable(); // IPv4 or IPv6
+    table.text('user_agent');
+    table.timestamp('attempt_time').defaultTo(knex.fn.now());
+    table.boolean('success').defaultTo(false);
+    
+    // Indexes for performance
+    table.index('identifier');
+    table.index('attempt_time');
+    table.index(['identifier', 'success', 'attempt_time']);
+  });
+};
+
+exports.down = function(knex) {
+  return knex.schema.dropTableIfExists('login_attempts');
+};