feat(exercises): enhance context in gap fill exercises and update migration for route descriptions

backend/services/vocabService.js

@@ -3617,7 +3617,7 @@ export default class VocabService {
     const seed = (Number(lessonId) * 100003) >>> 0;
     const percentage = 40 + (seed % 21);
     const targetCount = Math.max(1, Math.ceil((list.length * percentage) / 100));
-    return this._seededShuffle(list.slice(), seed).slice(0, targetCount);
+    return this._selectConcreteExamExercises(list, targetCount, seed);
   }
 
   _selectCheckpointExamExercises(exercises = [], lessonId) {
@@ -3628,7 +3628,73 @@ export default class VocabService {
     // Checkpoints: smaller sample ~10-30%
     const percentage = 10 + (seed % 21);
     const targetCount = Math.max(1, Math.ceil((list.length * percentage) / 100));
-    return this._seededShuffle(list.slice(), seed).slice(0, targetCount);
+    return this._selectConcreteExamExercises(list, targetCount, seed);
+  }
+
+  _getExamExerciseQuestionType(exercise) {
+    const questionData = typeof exercise?.questionData === 'string'
+      ? JSON.parse(exercise.questionData)
+      : (exercise?.questionData || {});
+    return String(questionData.type || exercise?.exerciseType?.name || '').trim().toLowerCase();
+  }
+
+  _isGenericPatternPrompt(exercise) {
+    const questionData = typeof exercise?.questionData === 'string'
+      ? JSON.parse(exercise.questionData)
+      : (exercise?.questionData || {});
+    const prompt = String(questionData.question || questionData.text || exercise?.instruction || '');
+    return /(?:kernmuster|zentrales? muster)/i.test(prompt);
+  }
+
+  /**
+   * Review and checkpoint exams must be concrete and reliably checkable.
+   * Prefer gap fills (one is included whenever available), then multiple
+   * choice, and use open "core pattern" prompts only when there is no other
+   * material available at all.
+   */
+  _selectConcreteExamExercises(exercises = [], targetCount, seed) {
+    const list = Array.isArray(exercises) ? exercises : [];
+    const groups = {
+      gap: [],
+      multipleChoice: [],
+      concreteOther: [],
+      fallback: []
+    };
+
+    for (const exercise of list) {
+      const type = this._getExamExerciseQuestionType(exercise);
+      if (type === 'gap_fill') {
+        groups.gap.push(exercise);
+      } else if (type === 'multiple_choice') {
+        groups.multipleChoice.push(exercise);
+      } else if (!this._isGenericPatternPrompt(exercise)
+        && !['speaking_from_memory', 'reading_aloud'].includes(type)) {
+        groups.concreteOther.push(exercise);
+      } else {
+        groups.fallback.push(exercise);
+      }
+    }
+
+    const selected = [];
+    const addUntilFull = (items, groupSeed) => {
+      for (const exercise of this._seededShuffle(items, groupSeed)) {
+        if (selected.length >= targetCount) break;
+        selected.push(exercise);
+      }
+    };
+
+    // A gap fill is a good, unambiguous opening question and avoids exams
+    // consisting only of abstract sentence-building prompts. Keep the rest
+    // balanced so a larger exam does not become a list of gaps only.
+    const shuffledGaps = this._seededShuffle(groups.gap, (seed ^ 0x1f123bb5) >>> 0);
+    if (shuffledGaps.length > 0) {
+      selected.push(shuffledGaps.shift());
+    }
+    addUntilFull(groups.multipleChoice, (seed ^ 0x39a1f8c7) >>> 0);
+    addUntilFull(shuffledGaps, (seed ^ 0x2d98c6f1) >>> 0);
+    addUntilFull(groups.concreteOther, (seed ^ 0x5bd1e995) >>> 0);
+    addUntilFull(groups.fallback, (seed ^ 0x7f4a7c15) >>> 0);
+    return selected;
   }
 
   /**