fix(usage): 用户使用记录不下发账号计费倍率

- 将 usage log DTO 拆分为用户/管理员两类
- 用户接口不返回 account_rate_multiplier/ip_address/account
- 管理员接口保留管理员字段
- 补充契约测试防止回归

backend/internal/handler/admin/usage_handler.go

@@ -163,7 +163,7 @@ func (h *UsageHandler) List(c *gin.Context) {
 		return
 	}
 
-	out := make([]dto.UsageLog, 0, len(records))
+	out := make([]dto.AdminUsageLog, 0, len(records))
 	for i := range records {
 		out = append(out, *dto.UsageLogFromServiceAdmin(&records[i]))
 	}

backend/internal/handler/dto/mappers.go

@@ -302,14 +302,9 @@ func AccountSummaryFromService(a *service.Account) *AccountSummary {
 	}
 }
 
-// usageLogFromServiceBase is a helper that converts service UsageLog to DTO.
-// The account parameter allows caller to control what Account info is included.
-// The includeIPAddress parameter controls whether to include the IP address (admin-only).
-func usageLogFromServiceBase(l *service.UsageLog, account *AccountSummary, includeIPAddress bool) *UsageLog {
-	if l == nil {
-		return nil
-	}
-	result := &UsageLog{
+func usageLogFromServiceUser(l *service.UsageLog) UsageLog {
+	// 普通用户 DTO：严禁包含管理员字段（例如 account_rate_multiplier、ip_address、account）。
+	return UsageLog{
 		ID:                    l.ID,
 		UserID:                l.UserID,
 		APIKeyID:              l.APIKeyID,
@@ -331,7 +326,6 @@ 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,
@@ -342,30 +336,33 @@ func usageLogFromServiceBase(l *service.UsageLog, account *AccountSummary, inclu
 		CreatedAt:             l.CreatedAt,
 		User:                  UserFromServiceShallow(l.User),
 		APIKey:                APIKeyFromService(l.APIKey),
-		Account:               account,
 		Group:                 GroupFromServiceShallow(l.Group),
 		Subscription:          UserSubscriptionFromService(l.Subscription),
 	}
-	// IP 地址仅对管理员可见
-	if includeIPAddress {
-		result.IPAddress = l.IPAddress
-	}
-	return result
 }
 
 // UsageLogFromService converts a service UsageLog to DTO for regular users.
 // It excludes Account details and IP address - users should not see these.
 func UsageLogFromService(l *service.UsageLog) *UsageLog {
-	return usageLogFromServiceBase(l, nil, false)
+	if l == nil {
+		return nil
+	}
+	u := usageLogFromServiceUser(l)
+	return &u
 }
 
 // UsageLogFromServiceAdmin converts a service UsageLog to DTO for admin users.
 // It includes minimal Account info (ID, Name only) and IP address.
-func UsageLogFromServiceAdmin(l *service.UsageLog) *UsageLog {
+func UsageLogFromServiceAdmin(l *service.UsageLog) *AdminUsageLog {
 	if l == nil {
 		return nil
 	}
-	return usageLogFromServiceBase(l, AccountSummaryFromService(l.Account), true)
+	return &AdminUsageLog{
+		UsageLog:              usageLogFromServiceUser(l),
+		AccountRateMultiplier: l.AccountRateMultiplier,
+		IPAddress:             l.IPAddress,
+		Account:               AccountSummaryFromService(l.Account),
+	}
 }
 
 func UsageCleanupTaskFromService(task *service.UsageCleanupTask) *UsageCleanupTask {

backend/internal/handler/dto/types.go

@@ -190,6 +190,7 @@ type RedeemCode struct {
 	Group *Group `json:"group,omitempty"`
 }
 
+// UsageLog 是普通用户接口使用的 usage log DTO（不包含管理员字段）。
 type UsageLog struct {
 	ID        int64  `json:"id"`
 	UserID    int64  `json:"user_id"`
@@ -209,14 +210,13 @@ 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"`
-	AccountRateMultiplier *float64 `json:"account_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"`
 
 	BillingType  int8 `json:"billing_type"`
 	Stream       bool `json:"stream"`
@@ -230,18 +230,28 @@ type UsageLog struct {
 	// User-Agent
 	UserAgent *string `json:"user_agent"`
 
-	// IP 地址（仅管理员可见）
-	IPAddress *string `json:"ip_address,omitempty"`
-
 	CreatedAt time.Time `json:"created_at"`
 
 	User         *User             `json:"user,omitempty"`
 	APIKey       *APIKey           `json:"api_key,omitempty"`
-	Account      *AccountSummary   `json:"account,omitempty"` // Use minimal AccountSummary to prevent data leakage
 	Group        *Group            `json:"group,omitempty"`
 	Subscription *UserSubscription `json:"subscription,omitempty"`
 }
 
+// AdminUsageLog 是管理员接口使用的 usage log DTO（包含管理员字段）。
+type AdminUsageLog struct {
+	UsageLog
+
+	// AccountRateMultiplier 账号计费倍率快照（nil 表示按 1.0 处理）
+	AccountRateMultiplier *float64 `json:"account_rate_multiplier"`
+
+	// IPAddress 用户请求 IP（仅管理员可见）
+	IPAddress *string `json:"ip_address,omitempty"`
+
+	// Account 最小账号信息（避免泄露敏感字段）
+	Account *AccountSummary `json:"account,omitempty"`
+}
+
 type UsageCleanupFilters struct {
 	StartTime   time.Time `json:"start_time"`
 	EndTime     time.Time `json:"end_time"`

backend/internal/server/api_contract_test.go

@@ -190,24 +190,25 @@ func TestAPIContracts(t *testing.T) {
 				t.Helper()
 				deps.usageRepo.SetUserLogs(1, []service.UsageLog{
 					{
-						ID:                  1,
-						UserID:              1,
-						APIKeyID:            100,
-						AccountID:           200,
-						RequestID:           "req_123",
-						Model:               "claude-3",
-						InputTokens:         10,
-						OutputTokens:        20,
-						CacheCreationTokens: 1,
-						CacheReadTokens:     2,
-						TotalCost:           0.5,
-						ActualCost:          0.5,
-						RateMultiplier:      1,
-						BillingType:         service.BillingTypeBalance,
-						Stream:              true,
-						DurationMs:          ptr(100),
-						FirstTokenMs:        ptr(50),
-						CreatedAt:           deps.now,
+						ID:                    1,
+						UserID:                1,
+						APIKeyID:              100,
+						AccountID:             200,
+						AccountRateMultiplier: ptr(0.5),
+						RequestID:             "req_123",
+						Model:                 "claude-3",
+						InputTokens:           10,
+						OutputTokens:          20,
+						CacheCreationTokens:   1,
+						CacheReadTokens:       2,
+						TotalCost:             0.5,
+						ActualCost:            0.5,
+						RateMultiplier:        1,
+						BillingType:           service.BillingTypeBalance,
+						Stream:                true,
+						DurationMs:            ptr(100),
+						FirstTokenMs:          ptr(50),
+						CreatedAt:             deps.now,
 					},
 				})
 			},
@@ -238,10 +239,9 @@ func TestAPIContracts(t *testing.T) {
 							"output_cost": 0,
 							"cache_creation_cost": 0,
 							"cache_read_cost": 0,
-							"total_cost": 0.5,
+						"total_cost": 0.5,
 						"actual_cost": 0.5,
 						"rate_multiplier": 1,
-						"account_rate_multiplier": null,
 						"billing_type": 0,
 							"stream": true,
 							"duration_ms": 100,