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

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

apps/api/database/migrations/20270101000058-create-announcement-reads.js

@@ -0,0 +1,43 @@
+'use strict';
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    await queryInterface.createTable('announcement_reads', {
+      id: {
+        type:          Sequelize.BIGINT,
+        autoIncrement: true,
+        primaryKey:    true,
+      },
+      announcement_id: {
+        type:       Sequelize.BIGINT,
+        allowNull:  false,
+        references: { model: 'announcements', key: 'id' },
+        onDelete:   'CASCADE',
+      },
+      user_id: {
+        type:       Sequelize.BIGINT,
+        allowNull:  false,
+        references: { model: 'users', key: 'user_id' },
+        onDelete:   'CASCADE',
+      },
+      read_at: {
+        type:         Sequelize.DATE,
+        allowNull:    false,
+        defaultValue: Sequelize.literal('CURRENT_TIMESTAMP'),
+      },
+    });
+
+    await queryInterface.addConstraint('announcement_reads', {
+      fields: ['announcement_id', 'user_id'],
+      type:   'unique',
+      name:   'uq_announcement_reads',
+    });
+
+    await queryInterface.addIndex('announcement_reads', { fields: ['announcement_id'], name: 'idx_announcement_reads_ann_id' });
+    await queryInterface.addIndex('announcement_reads', { fields: ['user_id'],         name: 'idx_announcement_reads_user_id' });
+  },
+
+  async down(queryInterface) {
+    await queryInterface.dropTable('announcement_reads');
+  },
+};