fix(backend): fix 500 error in mastery stats endpoint with robust error handling

backend/app/modules/quiz/router.py

@@ -551,6 +551,7 @@ async def get_mastery_stats(
     db: AsyncSession = Depends(get_async_session)
 ):
     """Calculates mastery level per topic."""
+    try:
         from app.modules.quiz.models import RemediationQueue
         
         # 1. Get all scores
@@ -568,6 +569,7 @@ async def get_mastery_stats(
         topic_map = {}
         
         for s in scores:
+            if not s.topic: continue
             # Normalize topic
             clean_topic = s.topic.replace(" (Remediation)", "").replace(" (Remédiation)", "").strip()
             
@@ -589,12 +591,15 @@ async def get_mastery_stats(
         
         error_counts = {}
         for e in errors:
+            if not e.topic: continue
             # Also clean topic for errors if needed, or rely on exact match? 
             # Ideally errors should also be grouped by clean topic.
             t_key = e.topic.replace(" (Remediation)", "").replace(" (Remédiation)", "").strip()
             error_counts[t_key] = error_counts.get(t_key, 0) + 1
 
         for topic, topic_scores in topic_map.items():
+            if not topic_scores: continue
+            
             # Sort by date asc (oldest first)
             topic_scores.sort(key=lambda x: x.created_at)
             
@@ -627,13 +632,12 @@ async def get_mastery_stats(
             elif final_mastery >= 50:
                 status = "REVIEWING"
                 
-        last_activity = topic_scores[-1].created_at
+            # Safely get last activity
+            last_activity = topic_scores[-1].created_at if topic_scores else datetime.utcnow()
             
             # Find the latest revision for this topic to get synthesis and tips
             latest_revision = None
-        # topic_scores are already sorted by date asc, so let's find latest revision
+            try:
-        # Actually, let's find the latest revision object directly.
-        # Revision might have a different topic name? No, normalization should match.
                 from app.modules.quiz.models import Revision
                 rev_stmt = select(Revision).where(
                     Revision.learner_id == learner_id,
@@ -641,6 +645,8 @@ async def get_mastery_stats(
                 ).order_by(Revision.created_at.desc()).limit(1)
                 rev_result = await db.execute(rev_stmt)
                 latest_revision = rev_result.scalar_one_or_none()
+            except Exception:
+                pass # Ignore revision fetch errors
 
             mastery_list.append({
                 "topic": topic,
@@ -657,6 +663,11 @@ async def get_mastery_stats(
         mastery_list.sort(key=lambda x: x['last_activity'], reverse=True)
         
         return mastery_list
+    except Exception as e:
+        import traceback
+        print(f"ERROR get_mastery_stats: {str(e)}\n{traceback.format_exc()}")
+        # Return empty list instead of 500 to keep dashboard alive
+        return []
 
 @router.get("/stats/activity")
 async def get_activity_stats(