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/index.js

@@ -158,6 +158,8 @@ import VocabGrammarExerciseType from './community/vocab_grammar_exercise_type.js
 import VocabGrammarExercise from './community/vocab_grammar_exercise.js';
 import VocabGrammarExerciseProgress from './community/vocab_grammar_exercise_progress.js';
 import VocabSrsItem from './community/vocab_srs_item.js';
+import VocabConversationSession from './community/vocab_conversation_session.js';
+import VocabListeningActivity from './community/vocab_listening_activity.js';
 import CalendarEvent from './community/calendar_event.js';
 
 const models = {
@@ -322,6 +324,8 @@ const models = {
   VocabGrammarExercise,
   VocabGrammarExerciseProgress,
   VocabSrsItem,
+  VocabConversationSession,
+  VocabListeningActivity,
   
   // Calendar
   CalendarEvent,