refactor: 后端日志系统重构

- 新增模块化日志器（pkg/logger/module.go）
- 新增 GORM 日志适配器
- 统一日志入口，移除所有 zap.L() 全局 logger 调用
- 字段标准化
- 启动阶段使用结构化日志
- 更新所有相关测试

backend/internal/handler/proxy_handler.go

@@ -16,6 +16,7 @@ import (
 	"nex/backend/internal/provider"
 	"nex/backend/internal/service"
 	"nex/backend/pkg/modelid"
+	pkglogger "nex/backend/pkg/logger"
 )
 
 // ProxyHandler 统一代理处理器
@@ -29,14 +30,14 @@ type ProxyHandler struct {
 }
 
 // NewProxyHandler 创建统一代理处理器
-func NewProxyHandler(engine *conversion.ConversionEngine, client provider.ProviderClient, routingService service.RoutingService, providerService service.ProviderService, statsService service.StatsService) *ProxyHandler {
+func NewProxyHandler(engine *conversion.ConversionEngine, client provider.ProviderClient, routingService service.RoutingService, providerService service.ProviderService, statsService service.StatsService, logger *zap.Logger) *ProxyHandler {
 	return &ProxyHandler{
 		engine:          engine,
 		client:          client,
 		routingService:  routingService,
 		providerService: providerService,
 		statsService:    statsService,
-		logger:          zap.L(),
+		logger:          pkglogger.WithModule(logger, "handler.proxy"),
 	}
 }
 
@@ -331,7 +332,7 @@ func (h *ProxyHandler) handleModelInfo(c *gin.Context, unifiedID string, adapter
 	// 使用 adapter 编码返回
 	body, err := adapter.EncodeModelInfoResponse(modelInfo)
 	if err != nil {
-		h.logger.Error("编码 ModelInfo 响应失败", zap.String("error", err.Error()))
+		h.logger.Error("编码 ModelInfo 响应失败", zap.Error(err))
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "编码响应失败"})
 		return
 	}