feat(vocab): add vocab distractor pool functionality

- Implemented a new endpoint to retrieve a pool of distractors for vocabulary exercises based on prior lessons.
- Updated the VocabController and VocabRouter to include the new getVocabDistractorPool method.
- Enhanced VocabService to classify questions and gather distractors from previous lessons.
- Modified VocabLessonView to fetch and utilize the distractor pool for multiple-choice exercises, improving the learning experience.

backend/controllers/vocabController.js

@@ -27,6 +27,9 @@ class VocabController {
     this.createCourse = this._wrapWithUser((userId, req) => this.service.createCourse(userId, req.body), { successStatus: 201 });
     this.getCourses = this._wrapWithUser((userId, req) => this.service.getCourses(userId, req.query));
     this.getCourse = this._wrapWithUser((userId, req) => this.service.getCourse(userId, req.params.courseId));
+    this.getVocabDistractorPool = this._wrapWithUser((userId, req) =>
+      this.service.getVocabDistractorPool(userId, req.params.courseId, req.query.beforeLessonId)
+    );
     this.getCourseByShareCode = this._wrapWithUser((userId, req) => this.service.getCourseByShareCode(userId, req.body.shareCode));
     this.updateCourse = this._wrapWithUser((userId, req) => this.service.updateCourse(userId, req.params.courseId, req.body));
     this.deleteCourse = this._wrapWithUser((userId, req) => this.service.deleteCourse(userId, req.params.courseId));