chore: relocate backend into apps/api ahead of monorepo merge

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/api/database/migrations/20270101000060-create-page-user-groups.js

@@ -0,0 +1,33 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.createTable('page_user_groups', {
+      page_id: {
+        type:       Sequelize.BIGINT,
+        allowNull:  false,
+        primaryKey: true,
+        references: { model: 'pages', key: 'id' },
+        onDelete:   'CASCADE',
+      },
+      group_id: {
+        type:       Sequelize.BIGINT,
+        allowNull:  false,
+        primaryKey: true,
+        references: { model: 'user_groups', key: 'group_id' },
+        onDelete:   'CASCADE',
+      },
+      created_at: {
+        type:         Sequelize.DATE,
+        allowNull:    false,
+        defaultValue: Sequelize.literal('CURRENT_TIMESTAMP'),
+      },
+    });
+
+    await queryInterface.addIndex('page_user_groups', { fields: ['group_id'], name: 'idx_page_user_groups_group_id' });
+  },
+
+  async down(queryInterface) {
+    await queryInterface.dropTable('page_user_groups');
+  },
+};