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/services/vocabService.js

@@ -1932,9 +1932,22 @@ export default class VocabService {
       }
       
       if (correctIndices.length === 0) return false;
-      
-      // userAnswer ist der Index (0, 1, 2, ...)
+
+      const correctTexts = correctIndices
+        .map((i) => options[i])
+        .filter((opt) => opt !== undefined && opt !== null);
+      const norm = (s) => String(s).trim().toLowerCase();
+
+      // Nach zufälligen Distraktoren: Client sendet gewählten Optionstext statt Index
+      if (typeof userAnswer === 'string') {
+        const u = norm(userAnswer);
+        if (!u) return false;
+        return correctTexts.some((t) => norm(t) === u);
+      }
+
+      // Legacy: Index in die gespeicherten (nicht gemischten) Optionen
       const userIndex = Number(userAnswer);
+      if (Number.isNaN(userIndex)) return false;
       return correctIndices.includes(userIndex);
     }
 
@@ -2083,4 +2096,88 @@ export default class VocabService {
     await exercise.destroy();
     return { success: true };
   }
+
+  /**
+   * Ordnet eine Multiple-Choice-Frage der Zielsprache (zu lernen) oder Muttersprache (Erklärung) zu,
+   * damit Distraktoren aus dem passenden Wortpool gewählt werden können.
+   * @returns {'target'|'native'|'unknown'}
+   */
+  _classifyMcQuestionSide(question) {
+    const q = String(question || '');
+    if (/Wie sagt man\s/i.test(q) || /Übersetze/i.test(q)) return 'target';
+    if (/Was bedeutet/i.test(q)) return 'native';
+    return 'unknown';
+  }
+
+  /**
+   * Sammelt Vokabeln aus allen Multiple-Choice-Übungen von Lektionen **vor** der angegebenen Lektion
+   * (gleicher Kurs), getrennt nach Ziel- vs. Muttersprache anhand der Frageformulierung.
+   */
+  async getVocabDistractorPool(hashedUserId, courseId, beforeLessonId) {
+    if (!beforeLessonId) {
+      const err = new Error('beforeLessonId is required');
+      err.status = 400;
+      throw err;
+    }
+    const user = await this._getUserByHashedId(hashedUserId);
+    const enrollment = await VocabCourseEnrollment.findOne({
+      where: { userId: user.id, courseId: Number(courseId) },
+    });
+    if (!enrollment) {
+      const err = new Error('Not enrolled in this course');
+      err.status = 403;
+      throw err;
+    }
+
+    const currentLesson = await VocabCourseLesson.findByPk(beforeLessonId);
+    if (!currentLesson || currentLesson.courseId !== Number(courseId)) {
+      const err = new Error('Lesson not found');
+      err.status = 404;
+      throw err;
+    }
+
+    const priorLessons = await VocabCourseLesson.findAll({
+      where: {
+        courseId: Number(courseId),
+        lessonNumber: { [Op.lt]: currentLesson.lessonNumber },
+      },
+      attributes: ['id'],
+      order: [['lessonNumber', 'ASC']],
+    });
+
+    const lessonIds = priorLessons.map((l) => l.id);
+    if (lessonIds.length === 0) {
+      return { target: [], native: [] };
+    }
+
+    const exercises = await VocabGrammarExercise.findAll({
+      where: {
+        lessonId: { [Op.in]: lessonIds },
+        exerciseTypeId: 2,
+      },
+      attributes: ['questionData'],
+    });
+
+    const target = new Set();
+    const native = new Set();
+
+    for (const ex of exercises) {
+      const qd =
+        typeof ex.questionData === 'string' ? JSON.parse(ex.questionData) : ex.questionData;
+      const question = qd?.question || '';
+      const opts = qd?.options;
+      if (!Array.isArray(opts)) continue;
+      const side = this._classifyMcQuestionSide(question);
+      if (side === 'target') {
+        opts.forEach((o) => target.add(String(o).trim()));
+      } else if (side === 'native') {
+        opts.forEach((o) => native.add(String(o).trim()));
+      }
+    }
+
+    return {
+      target: [...target],
+      native: [...native],
+    };
+  }
 }