feat(计费): 支持账号计费倍率快照与统计展示

- 新增 accounts.rate_multiplier（默认 1.0，允许 0）
- 使用 usage_logs.account_rate_multiplier 记录倍率快照，避免历史回算
- 统计/导出/管理端展示账号口径费用（total_cost * account_rate_multiplier）

backend/internal/service/admin_service.go

@@ -136,6 +136,7 @@ type CreateAccountInput struct {
 	ProxyID            *int64
 	Concurrency        int
 	Priority           int
+	RateMultiplier     *float64 // 账号计费倍率（>=0，允许 0）
 	GroupIDs           []int64
 	ExpiresAt          *int64
 	AutoPauseOnExpired *bool
@@ -151,8 +152,9 @@ type UpdateAccountInput struct {
 	Credentials           map[string]any
 	Extra                 map[string]any
 	ProxyID               *int64
-	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"
+	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）
 	Status                string
 	GroupIDs              *[]int64
 	ExpiresAt             *int64
@@ -162,16 +164,17 @@ type UpdateAccountInput struct {
 
 // BulkUpdateAccountsInput describes the payload for bulk updating accounts.
 type BulkUpdateAccountsInput struct {
-	AccountIDs  []int64
-	Name        string
-	ProxyID     *int64
-	Concurrency *int
-	Priority    *int
-	Status      string
-	Schedulable *bool
-	GroupIDs    *[]int64
-	Credentials map[string]any
-	Extra       map[string]any
+	AccountIDs     []int64
+	Name           string
+	ProxyID        *int64
+	Concurrency    *int
+	Priority       *int
+	RateMultiplier *float64 // 账号计费倍率（>=0，允许 0）
+	Status         string
+	Schedulable    *bool
+	GroupIDs       *[]int64
+	Credentials    map[string]any
+	Extra          map[string]any
 	// SkipMixedChannelCheck skips the mixed channel risk check when binding groups.
 	// This should only be set when the caller has explicitly confirmed the risk.
 	SkipMixedChannelCheck bool
@@ -817,6 +820,12 @@ func (s *adminServiceImpl) CreateAccount(ctx context.Context, input *CreateAccou
 	} else {
 		account.AutoPauseOnExpired = true
 	}
+	if input.RateMultiplier != nil {
+		if *input.RateMultiplier < 0 {
+			return nil, errors.New("rate_multiplier must be >= 0")
+		}
+		account.RateMultiplier = input.RateMultiplier
+	}
 	if err := s.accountRepo.Create(ctx, account); err != nil {
 		return nil, err
 	}
@@ -869,6 +878,12 @@ func (s *adminServiceImpl) UpdateAccount(ctx context.Context, id int64, input *U
 	if input.Priority != nil {
 		account.Priority = *input.Priority
 	}
+	if input.RateMultiplier != nil {
+		if *input.RateMultiplier < 0 {
+			return nil, errors.New("rate_multiplier must be >= 0")
+		}
+		account.RateMultiplier = input.RateMultiplier
+	}
 	if input.Status != "" {
 		account.Status = input.Status
 	}
@@ -942,6 +957,12 @@ func (s *adminServiceImpl) BulkUpdateAccounts(ctx context.Context, input *BulkUp
 		}
 	}
 
+	if input.RateMultiplier != nil {
+		if *input.RateMultiplier < 0 {
+			return nil, errors.New("rate_multiplier must be >= 0")
+		}
+	}
+
 	// Prepare bulk updates for columns and JSONB fields.
 	repoUpdates := AccountBulkUpdate{
 		Credentials: input.Credentials,
@@ -959,6 +980,9 @@ func (s *adminServiceImpl) BulkUpdateAccounts(ctx context.Context, input *BulkUp
 	if input.Priority != nil {
 		repoUpdates.Priority = input.Priority
 	}
+	if input.RateMultiplier != nil {
+		repoUpdates.RateMultiplier = input.RateMultiplier
+	}
 	if input.Status != "" {
 		repoUpdates.Status = &input.Status
 	}