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/tests/integration/e2e_conversion_test.go

@@ -61,10 +61,10 @@ func setupE2ETest(t *testing.T) (*gin.Engine, *httptest.Server) {
 	registry := conversion.NewMemoryRegistry()
 	require.NoError(t, registry.Register(openaiConv.NewAdapter()))
 	require.NoError(t, registry.Register(anthropic.NewAdapter()))
-	engine := conversion.NewConversionEngine(registry, nil)
+	engine := conversion.NewConversionEngine(registry, logger)
 
-	providerClient := provider.NewClient()
-	proxyHandler := handler.NewProxyHandler(engine, providerClient, routingService, providerService, statsService)
+	providerClient := provider.NewClient(logger)
+	proxyHandler := handler.NewProxyHandler(engine, providerClient, routingService, providerService, statsService, logger)
 	providerHandler := handler.NewProviderHandler(providerService)
 	modelHandler := handler.NewModelHandler(modelService)