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/admin_service.go

@@ -198,6 +198,7 @@ type CreateAccountInput struct {
 	Concurrency        int
 	Priority           int
 	RateMultiplier     *float64 // 账号计费倍率（>=0，允许 0）
+	LoadFactor         *int
 	GroupIDs           []int64
 	ExpiresAt          *int64
 	AutoPauseOnExpired *bool
@@ -218,6 +219,7 @@ type UpdateAccountInput struct {
 	Concurrency           *int     // 使用指针区分"未提供"和"设置为0"
 	Priority              *int     // 使用指针区分"未提供"和"设置为0"
 	RateMultiplier        *float64 // 账号计费倍率（>=0，允许 0）
+	LoadFactor            *int
 	Status                string
 	GroupIDs              *[]int64
 	ExpiresAt             *int64
@@ -233,6 +235,7 @@ type BulkUpdateAccountsInput struct {
 	Concurrency    *int
 	Priority       *int
 	RateMultiplier *float64 // 账号计费倍率（>=0，允许 0）
+	LoadFactor     *int
 	Status         string
 	Schedulable    *bool
 	GroupIDs       *[]int64
@@ -1433,6 +1436,9 @@ func (s *adminServiceImpl) CreateAccount(ctx context.Context, input *CreateAccou
 		}
 		account.RateMultiplier = input.RateMultiplier
 	}
+	if input.LoadFactor != nil && *input.LoadFactor > 0 {
+		account.LoadFactor = input.LoadFactor
+	}
 	if err := s.accountRepo.Create(ctx, account); err != nil {
 		return nil, err
 	}
@@ -1507,6 +1513,13 @@ func (s *adminServiceImpl) UpdateAccount(ctx context.Context, id int64, input *U
 		}
 		account.RateMultiplier = input.RateMultiplier
 	}
+	if input.LoadFactor != nil {
+		if *input.LoadFactor <= 0 {
+			account.LoadFactor = nil // 0 或负数表示清除
+		} else {
+			account.LoadFactor = input.LoadFactor
+		}
+	}
 	if input.Status != "" {
 		account.Status = input.Status
 	}
@@ -1640,6 +1653,9 @@ func (s *adminServiceImpl) BulkUpdateAccounts(ctx context.Context, input *BulkUp
 	if input.RateMultiplier != nil {
 		repoUpdates.RateMultiplier = input.RateMultiplier
 	}
+	if input.LoadFactor != nil {
+		repoUpdates.LoadFactor = input.LoadFactor
+	}
 	if input.Status != "" {
 		repoUpdates.Status = &input.Status
 	}