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/account_load_factor_test.go

@@ -1,3 +1,5 @@
+//go:build unit
+
 package service
 
 import (
@@ -6,6 +8,8 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func intPtrHelper(v int) *int { return &v }
+
 func TestEffectiveLoadFactor_NilAccount(t *testing.T) {
 	var a *Account
 	require.Equal(t, 1, a.EffectiveLoadFactor())
@@ -22,21 +26,21 @@ func TestEffectiveLoadFactor_NilLoadFactor_ZeroConcurrency(t *testing.T) {
 }
 
 func TestEffectiveLoadFactor_PositiveLoadFactor(t *testing.T) {
-	a := &Account{Concurrency: 5, LoadFactor: intPtr(20)}
+	a := &Account{Concurrency: 5, LoadFactor: intPtrHelper(20)}
 	require.Equal(t, 20, a.EffectiveLoadFactor())
 }
 
 func TestEffectiveLoadFactor_ZeroLoadFactor_FallbackToConcurrency(t *testing.T) {
-	a := &Account{Concurrency: 5, LoadFactor: intPtr(0)}
+	a := &Account{Concurrency: 5, LoadFactor: intPtrHelper(0)}
 	require.Equal(t, 5, a.EffectiveLoadFactor())
 }
 
 func TestEffectiveLoadFactor_NegativeLoadFactor_FallbackToConcurrency(t *testing.T) {
-	a := &Account{Concurrency: 3, LoadFactor: intPtr(-1)}
+	a := &Account{Concurrency: 3, LoadFactor: intPtrHelper(-1)}
 	require.Equal(t, 3, a.EffectiveLoadFactor())
 }
 
 func TestEffectiveLoadFactor_ZeroLoadFactor_ZeroConcurrency(t *testing.T) {
-	a := &Account{Concurrency: 0, LoadFactor: intPtr(0)}
+	a := &Account{Concurrency: 0, LoadFactor: intPtrHelper(0)}
 	require.Equal(t, 1, a.EffectiveLoadFactor())
 }