Fix bulk delete foreign key constraints and bulk upload phone unique constraint

backend/main.py

@@ -231,7 +231,7 @@ def change_password(payload: ChangePasswordRequest, session: Session = Depends(g
 async def bulk_register_students(file: UploadFile = File(...), session: Session = Depends(get_session)):
     # Simple check for now (we can rely on frontend calling it correctly or auth middleware if we add it)
     content = await file.read()
-    decoded = content.decode('utf-8')
+    decoded = content.decode('utf-8-sig')
     csv_reader = csv.DictReader(io.StringIO(decoded))
     
     results = []
@@ -267,7 +267,7 @@ async def bulk_register_students(file: UploadFile = File(...), session: Session
         user = models.User(
             email=email,
             name=name,
-            phone=phone,
+            phone=phone if phone else None,
             password_hash=hashed_password,
             target_exam=exam,
             tier=tier,
@@ -297,6 +297,9 @@ def bulk_delete_students(payload: DeleteUsersRequest, session: Session = Depends
             # Delete associated records manually since foreign key cascade might not be configured in sqlmodel by default
             session.exec(delete(models.Attempt).where(models.Attempt.user_id == uid))
             session.exec(delete(models.Mistake).where(models.Mistake.user_id == uid))
+            session.exec(delete(models.Transaction).where(models.Transaction.user_id == uid))
+            session.exec(delete(models.Subscription).where(models.Subscription.user_id == uid))
+            session.exec(delete(models.StudyPlan).where(models.StudyPlan.user_id == uid))
             session.delete(user)
     session.commit()
     return {"success": True}