refactor(dto): split admin usage upstream model exposure

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/handler/dto/mappers.go

@@ -528,8 +528,7 @@ func usageLogFromServiceUser(l *service.UsageLog) UsageLog {
 		APIKeyID:              l.APIKeyID,
 		AccountID:             l.AccountID,
 		RequestID:             l.RequestID,
-		Model:                 l.Model,
-		UpstreamModel:         l.UpstreamModel,
+		Model:                 l.RequestedModel,
 		ServiceTier:           l.ServiceTier,
 		ReasoningEffort:       l.ReasoningEffort,
 		InboundEndpoint:       l.InboundEndpoint,
@@ -586,6 +585,7 @@ func UsageLogFromServiceAdmin(l *service.UsageLog) *AdminUsageLog {
 	}
 	return &AdminUsageLog{
 		UsageLog:              usageLogFromServiceUser(l),
+		UpstreamModel:         l.UpstreamModel,
 		AccountRateMultiplier: l.AccountRateMultiplier,
 		IPAddress:             l.IPAddress,
 		Account:               AccountSummaryFromService(l.Account),

backend/internal/handler/dto/mappers_usage_test.go

@@ -1,6 +1,7 @@
 package dto
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
@@ -106,6 +107,32 @@ func TestUsageLogFromService_IncludesServiceTierForUserAndAdmin(t *testing.T) {
 	require.InDelta(t, 1.5, *adminDTO.AccountRateMultiplier, 1e-12)
 }
 
+func TestUsageLogFromService_UsesRequestedModelAndKeepsUpstreamAdminOnly(t *testing.T) {
+	t.Parallel()
+
+	upstreamModel := "claude-sonnet-4-20250514"
+	log := &service.UsageLog{
+		RequestID:      "req_4",
+		Model:          upstreamModel,
+		RequestedModel: "claude-sonnet-4",
+		UpstreamModel:  &upstreamModel,
+	}
+
+	userDTO := UsageLogFromService(log)
+	adminDTO := UsageLogFromServiceAdmin(log)
+
+	require.Equal(t, "claude-sonnet-4", userDTO.Model)
+	require.Equal(t, "claude-sonnet-4", adminDTO.Model)
+
+	userJSON, err := json.Marshal(userDTO)
+	require.NoError(t, err)
+	require.NotContains(t, string(userJSON), "upstream_model")
+
+	adminJSON, err := json.Marshal(adminDTO)
+	require.NoError(t, err)
+	require.Contains(t, string(adminJSON), `"upstream_model":"claude-sonnet-4-20250514"`)
+}
+
 func f64Ptr(value float64) *float64 {
 	return &value
 }

backend/internal/handler/dto/types.go

@@ -334,9 +334,6 @@ type UsageLog struct {
 	AccountID int64  `json:"account_id"`
 	RequestID string `json:"request_id"`
 	Model     string `json:"model"`
-	// UpstreamModel is the actual model sent to the upstream provider after mapping.
-	// Omitted when no mapping was applied (requested model was used as-is).
-	UpstreamModel *string `json:"upstream_model,omitempty"`
 	// ServiceTier records the OpenAI service tier used for billing, e.g. "priority" / "flex".
 	ServiceTier *string `json:"service_tier,omitempty"`
 	// ReasoningEffort is the request's reasoning effort level.
@@ -396,6 +393,10 @@ type UsageLog struct {
 type AdminUsageLog struct {
 	UsageLog
 
+	// UpstreamModel is the actual model sent to the upstream provider after mapping.
+	// Omitted when no mapping was applied (requested model was used as-is).
+	UpstreamModel *string `json:"upstream_model,omitempty"`
+
 	// AccountRateMultiplier 账号计费倍率快照（nil 表示按 1.0 处理）
 	AccountRateMultiplier *float64 `json:"account_rate_multiplier"`