feat: add independent load_factor field for scheduling load calculation

- Separate load factor from concurrency: concurrency controls actual
  slot acquisition, load_factor controls load rate calculation
- Add EffectiveLoadFactor() method: LoadFactor > Concurrency > 1
- Add load_factor field to Create/Edit/BulkEdit account forms
- Fix RPM default value: auto-fill 15 when RPM enabled but not set
- Fix stale test compilation errors in server and handler packages

backend/internal/service/account_load_factor_test.go

@@ -0,0 +1,42 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestEffectiveLoadFactor_NilAccount(t *testing.T) {
+	var a *Account
+	require.Equal(t, 1, a.EffectiveLoadFactor())
+}
+
+func TestEffectiveLoadFactor_NilLoadFactor_PositiveConcurrency(t *testing.T) {
+	a := &Account{Concurrency: 5}
+	require.Equal(t, 5, a.EffectiveLoadFactor())
+}
+
+func TestEffectiveLoadFactor_NilLoadFactor_ZeroConcurrency(t *testing.T) {
+	a := &Account{Concurrency: 0}
+	require.Equal(t, 1, a.EffectiveLoadFactor())
+}
+
+func TestEffectiveLoadFactor_PositiveLoadFactor(t *testing.T) {
+	a := &Account{Concurrency: 5, LoadFactor: intPtr(20)}
+	require.Equal(t, 20, a.EffectiveLoadFactor())
+}
+
+func TestEffectiveLoadFactor_ZeroLoadFactor_FallbackToConcurrency(t *testing.T) {
+	a := &Account{Concurrency: 5, LoadFactor: intPtr(0)}
+	require.Equal(t, 5, a.EffectiveLoadFactor())
+}
+
+func TestEffectiveLoadFactor_NegativeLoadFactor_FallbackToConcurrency(t *testing.T) {
+	a := &Account{Concurrency: 3, LoadFactor: intPtr(-1)}
+	require.Equal(t, 3, a.EffectiveLoadFactor())
+}
+
+func TestEffectiveLoadFactor_ZeroLoadFactor_ZeroConcurrency(t *testing.T) {
+	a := &Account{Concurrency: 0, LoadFactor: intPtr(0)}
+	require.Equal(t, 1, a.EffectiveLoadFactor())
+}