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_course.js

@@ -39,6 +39,22 @@ VocabCourse.init({
         defaultValue: 1,
         field: 'difficulty_level'
     },
+    cefrLevel: {
+        type: DataTypes.TEXT,
+        allowNull: true,
+        field: 'cefr_level'
+    },
+    courseKind: {
+        type: DataTypes.TEXT,
+        allowNull: false,
+        defaultValue: 'core',
+        field: 'course_kind'
+    },
+    prerequisiteCourseId: {
+        type: DataTypes.INTEGER,
+        allowNull: true,
+        field: 'prerequisite_course_id'
+    },
     isPublic: {
         type: DataTypes.BOOLEAN,
         allowNull: false,