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/pkg/logger/context.go

@@ -1,13 +1,20 @@
 package logger
 
-import "go.uber.org/zap"
+import (
+	"context"
+
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+)
+
+type ctxKey struct{}
+
+const requestIDKey = "request_id"
 
-// WithRequestID 向 logger 添加 request_id 字段
 func WithRequestID(logger *zap.Logger, requestID string) *zap.Logger {
-	return logger.With(zap.String("request_id", requestID))
+	return logger.With(zap.String(requestIDKey, requestID))
 }
 
-// WithContext 向 logger 添加多个自定义字段
 func WithContext(logger *zap.Logger, fields map[string]interface{}) *zap.Logger {
 	zapFields := make([]zap.Field, 0, len(fields))
 	for k, v := range fields {
@@ -15,3 +22,37 @@ func WithContext(logger *zap.Logger, fields map[string]interface{}) *zap.Logger
 	}
 	return logger.With(zapFields...)
 }
+
+func RequestIDFromGinContext(c *gin.Context) zap.Field {
+	requestID, exists := c.Get("request_id")
+	if !exists {
+		return zap.Skip()
+	}
+	if id, ok := requestID.(string); ok {
+		return RequestID(id)
+	}
+	return zap.Skip()
+}
+
+func RequestIDFromContext(ctx context.Context) zap.Field {
+	requestID := ctx.Value(ctxKey{})
+	if requestID == nil {
+		return zap.Skip()
+	}
+	if id, ok := requestID.(string); ok {
+		return RequestID(id)
+	}
+	return zap.Skip()
+}
+
+func ContextWithRequestID(ctx context.Context, requestID string) context.Context {
+	return context.WithValue(ctx, ctxKey{}, requestID)
+}
+
+func LoggerFromContext(ctx context.Context, baseLogger *zap.Logger) *zap.Logger {
+	field := RequestIDFromContext(ctx)
+	if field == zap.Skip() {
+		return baseLogger
+	}
+	return baseLogger.With(field)
+}