feat(backend): Go 服务骨架（配置、日志、数据库、中间件、Wire）

- config: Viper YAML 配置 + 环境变量覆盖
- logs: zap 结构化日志 + trace_id 链路追踪 + 敏感信息脱敏
- db: PostgreSQL (GORM) + Redis (go-redis) 连接管理
- middleware: Trace/Logger/CORS/Recovery 四层中间件
- provider: Wire 编译时依赖注入
- main.go: 优雅启动/关闭 HTTP 服务

Made-with: Cursor

backend/go-service/pkg/middleware/logger.go

@@ -0,0 +1,46 @@
+package middleware
+
+import (
+	"time"
+
+	"github.com/echochat/backend/pkg/logs"
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+)
+
+// Logger 请求日志中间件
+// 记录每个请求的完整信息：方法、路径、状态码、耗时、IP、User-Agent
+// 自动携带 trace_id，慢请求（>500ms）记录 WARN
+func Logger() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		start := time.Now()
+
+		c.Next()
+
+		latency := time.Since(start)
+		ctx := c.Request.Context()
+		funcName := "middleware.Logger"
+
+		fields := []zap.Field{
+			zap.String("method", c.Request.Method),
+			zap.String("path", c.Request.URL.Path),
+			zap.Int("status", c.Writer.Status()),
+			zap.Duration("latency", latency),
+			zap.String("ip", c.ClientIP()),
+			zap.String("user_agent", c.Request.UserAgent()),
+		}
+
+		if len(c.Errors) > 0 {
+			fields = append(fields, zap.String("error", c.Errors.String()))
+			logs.Error(ctx, funcName, "请求处理异常", fields...)
+			return
+		}
+
+		if latency > 500*time.Millisecond {
+			logs.Warn(ctx, funcName, "慢请求告警", fields...)
+			return
+		}
+
+		logs.Info(ctx, funcName, "请求完成", fields...)
+	}
+}