feat(exercises): enhance gap exercise creation with contextual prompts and add migration for lesson 43

backend/migrations-active/20260826000000-repair-lesson-43-context-gap.cjs

@@ -0,0 +1,28 @@
+'use strict';
+
+const LESSON_TITLE = 'Kultur: Höflichkeit, Familie, Alltag';
+
+module.exports = {
+  async up(queryInterface) {
+    await queryInterface.sequelize.transaction(async (transaction) => {
+      // The old task asked for a soft refusal to an invitation, but expected
+      // the unrelated cultural keyword "respeto". Test the matching phrase.
+      await queryInterface.sequelize.query(`
+        UPDATE community.vocab_grammar_exercise AS exercise
+        SET question_data = '{"type":"gap_fill","text":"{gap}","gaps":1}'::jsonb,
+            answer_data = '{"type":"gap_fill","answers":["Dili lang sa karon."]}'::jsonb,
+            explanation = 'Das Kernmuster lautet: "Dili lang sa karon."'
+        FROM community.vocab_course_lesson AS lesson
+        WHERE exercise.lesson_id = lesson.id
+          AND (lesson.id = 40 OR (lesson.course_id = 1 AND lesson.lesson_number = 43))
+          AND exercise.title = :lessonTitle || ': Kernmuster ergänzen'
+          AND exercise.question_data->>'type' = 'gap_fill'
+          AND exercise.answer_data->'answers' @> '["respeto"]'::jsonb;
+      `, { replacements: { lessonTitle: LESSON_TITLE }, transaction });
+    });
+  },
+
+  async down() {
+    // Do not restore the semantically mismatched task.
+  }
+};