feat: comment to test run's case (#390)

backend/migrations/20260131000000-create-comments.js

@@ -0,0 +1,47 @@
+export async function up(queryInterface, Sequelize) {
+  await queryInterface.createTable('comments', {
+    id: {
+      type: Sequelize.INTEGER,
+      primaryKey: true,
+      autoIncrement: true,
+    },
+    commentableType: {
+      type: Sequelize.STRING,
+      allowNull: false,
+    },
+    commentableId: {
+      type: Sequelize.INTEGER,
+      allowNull: false,
+    },
+    userId: {
+      type: Sequelize.INTEGER,
+      references: {
+        model: 'users',
+        key: 'id',
+      },
+      onUpdate: 'CASCADE',
+      onDelete: 'SET NULL',
+    },
+    content: {
+      type: Sequelize.TEXT,
+      allowNull: false,
+    },
+    createdAt: {
+      type: Sequelize.DATE,
+      allowNull: false,
+    },
+    updatedAt: {
+      type: Sequelize.DATE,
+      allowNull: false,
+    },
+  });
+
+  // Add composite index for efficient polymorphic queries
+  await queryInterface.addIndex('comments', ['commentableType', 'commentableId'], {
+    name: 'comments_commentable_index',
+  });
+}
+
+export async function down(queryInterface) {
+  await queryInterface.dropTable('comments');
+}