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

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

apps/api/database/migrations/20270101000057-create-announcement-roles.js

@@ -0,0 +1,35 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.createTable('announcement_roles', {
+      id: {
+        type:          Sequelize.BIGINT,
+        autoIncrement: true,
+        primaryKey:    true,
+      },
+      announcement_id: {
+        type:       Sequelize.BIGINT,
+        allowNull:  false,
+        references: { model: 'announcements', key: 'id' },
+        onDelete:   'CASCADE',
+      },
+      role: {
+        type:      Sequelize.STRING(50),
+        allowNull: false,
+      },
+    });
+
+    await queryInterface.addConstraint('announcement_roles', {
+      fields: ['announcement_id', 'role'],
+      type:   'unique',
+      name:   'uq_announcement_roles',
+    });
+
+    await queryInterface.addIndex('announcement_roles', { fields: ['announcement_id'], name: 'idx_announcement_roles_ann_id' });
+  },
+
+  async down(queryInterface) {
+    await queryInterface.dropTable('announcement_roles');
+  },
+};