add changes

models/UserBookmark.js

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+/**
+ * UserBookmark Model
+ * Junction table for user-saved questions
+ */
+
+const { DataTypes } = require('sequelize');
+const { v4: uuidv4 } = require('uuid');
+
+module.exports = (sequelize) => {
+  const UserBookmark = sequelize.define('UserBookmark', {
+    id: {
+      type: DataTypes.UUID,
+      defaultValue: () => uuidv4(),
+      primaryKey: true,
+      allowNull: false,
+      comment: 'Primary key UUID'
+    },
+    userId: {
+      type: DataTypes.UUID,
+      allowNull: false,
+      references: {
+        model: 'users',
+        key: 'id'
+      },
+      onDelete: 'CASCADE',
+      comment: 'Reference to user who bookmarked'
+    },
+    questionId: {
+      type: DataTypes.UUID,
+      allowNull: false,
+      references: {
+        model: 'questions',
+        key: 'id'
+      },
+      onDelete: 'CASCADE',
+      comment: 'Reference to bookmarked question'
+    },
+    notes: {
+      type: DataTypes.TEXT,
+      allowNull: true,
+      comment: 'Optional notes about the bookmark'
+    },
+    createdAt: {
+      type: DataTypes.DATE,
+      allowNull: false,
+      defaultValue: DataTypes.NOW,
+      field: 'created_at',
+      comment: 'When the bookmark was created'
+    },
+    updatedAt: {
+      type: DataTypes.DATE,
+      allowNull: false,
+      defaultValue: DataTypes.NOW,
+      field: 'updated_at',
+      comment: 'When the bookmark was last updated'
+    }
+  }, {
+    tableName: 'user_bookmarks',
+    timestamps: true,
+    underscored: true,
+    indexes: [
+      {
+        unique: true,
+        fields: ['user_id', 'question_id'],
+        name: 'idx_user_question_unique'
+      },
+      {
+        fields: ['user_id'],
+        name: 'idx_user_bookmarks_user'
+      },
+      {
+        fields: ['question_id'],
+        name: 'idx_user_bookmarks_question'
+      },
+      {
+        fields: ['bookmarked_at'],
+        name: 'idx_user_bookmarks_date'
+      }
+    ]
+  });
+
+  // Define associations
+  UserBookmark.associate = function(models) {
+    UserBookmark.belongsTo(models.User, {
+      foreignKey: 'userId',
+      as: 'User'
+    });
+    
+    UserBookmark.belongsTo(models.Question, {
+      foreignKey: 'questionId',
+      as: 'Question'
+    });
+  };
+
+  return UserBookmark;
+};