fix: address load_factor code review findings

- Fix bulk edit: send 0 instead of null/NaN to clear load_factor
- Fix edit modal: explicit NaN check instead of implicit falsy
- Fix create modal: use ?? instead of || for load_factor
- Add load_factor upper limit validation (max 10000)
- Add //go:build unit tag and self-contained intPtrHelper in test
- Add design intent comments on WaitPlan.MaxConcurrency

backend/internal/service/admin_service.go

@@ -1417,6 +1417,9 @@ func (s *adminServiceImpl) CreateAccount(ctx context.Context, input *CreateAccou
 		account.RateMultiplier = input.RateMultiplier
 	}
 	if input.LoadFactor != nil && *input.LoadFactor > 0 {
+		if *input.LoadFactor > 10000 {
+			return nil, errors.New("load_factor must be <= 10000")
+		}
 		account.LoadFactor = input.LoadFactor
 	}
 	if err := s.accountRepo.Create(ctx, account); err != nil {
@@ -1492,6 +1495,8 @@ func (s *adminServiceImpl) UpdateAccount(ctx context.Context, id int64, input *U
 	if input.LoadFactor != nil {
 		if *input.LoadFactor <= 0 {
 			account.LoadFactor = nil // 0 或负数表示清除
+		} else if *input.LoadFactor > 10000 {
+			return nil, errors.New("load_factor must be <= 10000")
 		} else {
 			account.LoadFactor = input.LoadFactor
 		}