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/handler/dto/mappers.go

@@ -125,6 +125,7 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 		ProxyID:                 a.ProxyID,
 		Concurrency:             a.Concurrency,
 		Priority:                a.Priority,
+		RateMultiplier:          a.BillingRateMultiplier(),
 		Status:                  a.Status,
 		ErrorMessage:            a.ErrorMessage,
 		LastUsedAt:              a.LastUsedAt,
@@ -279,6 +280,7 @@ func usageLogFromServiceBase(l *service.UsageLog, account *AccountSummary, inclu
 		TotalCost:             l.TotalCost,
 		ActualCost:            l.ActualCost,
 		RateMultiplier:        l.RateMultiplier,
+		AccountRateMultiplier: l.AccountRateMultiplier,
 		BillingType:           l.BillingType,
 		Stream:                l.Stream,
 		DurationMs:            l.DurationMs,

backend/internal/handler/dto/types.go

@@ -76,6 +76,7 @@ type Account struct {
 	ProxyID            *int64         `json:"proxy_id"`
 	Concurrency        int            `json:"concurrency"`
 	Priority           int            `json:"priority"`
+	RateMultiplier     float64        `json:"rate_multiplier"`
 	Status             string         `json:"status"`
 	ErrorMessage       string         `json:"error_message"`
 	LastUsedAt         *time.Time     `json:"last_used_at"`
@@ -169,13 +170,14 @@ type UsageLog struct {
 	CacheCreation5mTokens int `json:"cache_creation_5m_tokens"`
 	CacheCreation1hTokens int `json:"cache_creation_1h_tokens"`
 
-	InputCost         float64 `json:"input_cost"`
-	OutputCost        float64 `json:"output_cost"`
-	CacheCreationCost float64 `json:"cache_creation_cost"`
-	CacheReadCost     float64 `json:"cache_read_cost"`
-	TotalCost         float64 `json:"total_cost"`
-	ActualCost        float64 `json:"actual_cost"`
-	RateMultiplier    float64 `json:"rate_multiplier"`
+	InputCost             float64  `json:"input_cost"`
+	OutputCost            float64  `json:"output_cost"`
+	CacheCreationCost     float64  `json:"cache_creation_cost"`
+	CacheReadCost         float64  `json:"cache_read_cost"`
+	TotalCost             float64  `json:"total_cost"`
+	ActualCost            float64  `json:"actual_cost"`
+	RateMultiplier        float64  `json:"rate_multiplier"`
+	AccountRateMultiplier *float64 `json:"account_rate_multiplier"`
 
 	BillingType  int8 `json:"billing_type"`
 	Stream       bool `json:"stream"`