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.go

@@ -28,6 +28,7 @@ type Account struct {
 	// RateMultiplier 账号计费倍率（>=0，允许 0 表示该账号计费为 0）。
 	// 使用指针用于兼容旧版本调度缓存（Redis）中缺字段的情况：nil 表示按 1.0 处理。
 	RateMultiplier     *float64
+	LoadFactor         *int // 调度负载因子；nil 表示使用 Concurrency
 	Status             string
 	ErrorMessage       string
 	LastUsedAt         *time.Time
@@ -88,6 +89,19 @@ func (a *Account) BillingRateMultiplier() float64 {
 	return *a.RateMultiplier
 }
 
+func (a *Account) EffectiveLoadFactor() int {
+	if a == nil {
+		return 1
+	}
+	if a.LoadFactor != nil && *a.LoadFactor > 0 {
+		return *a.LoadFactor
+	}
+	if a.Concurrency > 0 {
+		return a.Concurrency
+	}
+	return 1
+}
+
 func (a *Account) IsSchedulable() bool {
 	if !a.IsActive() || !a.Schedulable {
 		return false