feat: 网关请求头 wire casing 保持、转发行为开关、调试日志增强及 accept-encoding 恢复

- 新增 header_util.go，通过 setHeaderRaw/getHeaderRaw/addHeaderRaw 绕过
  Go 的 canonical-case 规范化，保持真实 Claude CLI 抓包的请求头大小写
  （如 "x-app" 而非 "X-App"，"X-Stainless-OS" 而非 "X-Stainless-Os"）
- 新增管理后台开关：指纹统一化（默认开启）和 metadata 透传（默认关闭），
  使用 atomic.Value + singleflight 缓存模式，60s TTL
- 调试日志从控制台 body 打印升级为文件级完整快照
  （按真实 wire 顺序输出 headers + 格式化 JSON body + 上下文元数据）
- 恢复 accept-encoding 到白名单，在 http_upstream.go 新增 decompressResponseBody
  处理 gzip/brotli/deflate 解压（Go 显式设置 Accept-Encoding 时不会自动解压）
- OAuth 服务 axios UA 从 1.8.4 更新至 1.13.6
- 测试断言改用 getHeaderRaw 适配 raw header 存储方式

backend/internal/handler/admin/setting_handler.go

@@ -129,6 +129,8 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		MaxClaudeCodeVersion:                 settings.MaxClaudeCodeVersion,
 		AllowUngroupedKeyScheduling:          settings.AllowUngroupedKeyScheduling,
 		BackendModeEnabled:                   settings.BackendModeEnabled,
+		EnableFingerprintUnification:         settings.EnableFingerprintUnification,
+		EnableMetadataPassthrough:            settings.EnableMetadataPassthrough,
 	})
 }
 
@@ -209,6 +211,10 @@ type UpdateSettingsRequest struct {
 
 	// Backend Mode
 	BackendModeEnabled bool `json:"backend_mode_enabled"`
+
+	// Gateway forwarding behavior
+	EnableFingerprintUnification *bool `json:"enable_fingerprint_unification"`
+	EnableMetadataPassthrough    *bool `json:"enable_metadata_passthrough"`
 }
 
 // UpdateSettings 更新系统设置
@@ -601,6 +607,18 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 			}
 			return previousSettings.OpsMetricsIntervalSeconds
 		}(),
+		EnableFingerprintUnification: func() bool {
+			if req.EnableFingerprintUnification != nil {
+				return *req.EnableFingerprintUnification
+			}
+			return previousSettings.EnableFingerprintUnification
+		}(),
+		EnableMetadataPassthrough: func() bool {
+			if req.EnableMetadataPassthrough != nil {
+				return *req.EnableMetadataPassthrough
+			}
+			return previousSettings.EnableMetadataPassthrough
+		}(),
 	}
 
 	if err := h.settingService.UpdateSettings(c.Request.Context(), settings); err != nil {
@@ -679,6 +697,8 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		MaxClaudeCodeVersion:                 updatedSettings.MaxClaudeCodeVersion,
 		AllowUngroupedKeyScheduling:          updatedSettings.AllowUngroupedKeyScheduling,
 		BackendModeEnabled:                   updatedSettings.BackendModeEnabled,
+		EnableFingerprintUnification:         updatedSettings.EnableFingerprintUnification,
+		EnableMetadataPassthrough:            updatedSettings.EnableMetadataPassthrough,
 	})
 }
 
@@ -851,6 +871,12 @@ func diffSettings(before *service.SystemSettings, after *service.SystemSettings,
 	if before.CustomMenuItems != after.CustomMenuItems {
 		changed = append(changed, "custom_menu_items")
 	}
+	if before.EnableFingerprintUnification != after.EnableFingerprintUnification {
+		changed = append(changed, "enable_fingerprint_unification")
+	}
+	if before.EnableMetadataPassthrough != after.EnableMetadataPassthrough {
+		changed = append(changed, "enable_metadata_passthrough")
+	}
 	return changed
 }