feat: add A2-B1 course expansion features including listening activities and conversation sessions

- Updated VocabCourseView.vue to display CEFR level for courses.
- Enhanced VocabLessonView.vue with new sections for listening activities and conversation labs, including audio controls and transcript toggling.
- Created migration to add new columns to vocab_course and new tables for conversation sessions and listening activities.
- Implemented models for vocab_conversation_session and vocab_listening_activity.
- Developed scripts to create new A2 and B1 bridge courses with structured lesson plans.
- Documented the A2-B1 expansion plan detailing course structure, learning goals, and assessment criteria.

backend/models/community/vocab_conversation_session.js

@@ -0,0 +1,25 @@
+import { Model, DataTypes } from 'sequelize';
+import { sequelize } from '../../utils/sequelize.js';
+
+class VocabConversationSession extends Model {}
+
+VocabConversationSession.init({
+  id: { type: DataTypes.INTEGER, primaryKey: true, autoIncrement: true },
+  userId: { type: DataTypes.INTEGER, allowNull: false, field: 'user_id' },
+  lessonId: { type: DataTypes.INTEGER, allowNull: false, field: 'lesson_id' },
+  scenario: { type: DataTypes.JSONB, allowNull: false, defaultValue: {} },
+  turns: { type: DataTypes.JSONB, allowNull: false, defaultValue: [] },
+  summary: { type: DataTypes.JSONB, allowNull: true },
+  status: { type: DataTypes.TEXT, allowNull: false, defaultValue: 'active' },
+  createdAt: { type: DataTypes.DATE, allowNull: false, defaultValue: DataTypes.NOW, field: 'created_at' },
+  updatedAt: { type: DataTypes.DATE, allowNull: false, defaultValue: DataTypes.NOW, field: 'updated_at' }
+}, {
+  sequelize,
+  modelName: 'VocabConversationSession',
+  tableName: 'vocab_conversation_session',
+  schema: 'community',
+  timestamps: true,
+  underscored: true
+});
+
+export default VocabConversationSession;