Fix curriculum grounding for Grade 10: replace calculus fallback with authentic Systems of Equations questions and add self-healing guard

backend/app/Controllers/VideoController.php

@@ -423,6 +423,29 @@ class VideoController
 
         $lessonId = (int)$lesson['id'];
 
+        // Self-Healing Curriculum Guard: Purge any obsolete/mismatched calculus questions
+        // for non-calculus lessons (e.g. Grade 10 Systems of Equations)
+        $isCalculusLesson = (str_contains($lesson['title'], 'اشتقاق') || str_contains($lesson['title'], 'تفاضل'));
+        if (!$isCalculusLesson) {
+            try {
+                $mismatched = Database::selectOne(
+                    "SELECT q.id FROM questions q 
+                     JOIN exams e ON q.exam_id = e.id 
+                     WHERE e.lesson_id = ? AND (q.question_text LIKE '%مشتق%' OR q.question_text LIKE '%f\'(x)%')
+                     LIMIT 1",
+                    [$lessonId]
+                );
+                if ($mismatched) {
+                    $badExams = Database::select("SELECT id FROM exams WHERE lesson_id = ? AND scope = 'in_video_checkpoint'", [$lessonId]);
+                    foreach ($badExams as $be) {
+                        Database::query("DELETE FROM exams WHERE id = ?", [$be['id']]);
+                    }
+                }
+            } catch (\Throwable $e) {
+                error_log("Curriculum self-healing notice: " . $e->getMessage());
+            }
+        }
+
         // If lesson has no checkpoints or missing questions, generate them autonomously
         $existingCount = Database::selectOne("SELECT COUNT(*) as cnt FROM exams WHERE lesson_id = ? AND scope = 'in_video_checkpoint'", [$lessonId]);
         $existingQuestions = Database::selectOne("SELECT COUNT(*) as cnt FROM questions q JOIN exams e ON q.exam_id = e.id WHERE e.lesson_id = ?", [$lessonId]);