feat(middleware): 新建请求日志与 Panic 恢复中间件

- Logger 中间件：生成 request_id（注入 X-Request-ID header），记录 method/path/status/latency/client_ip
- Recovery 中间件：自定义 panic 恢复，记录 request 上下文和堆栈

backend/internal/mildware/logger.go

@@ -0,0 +1,81 @@
+package mildware
+
+import (
+	"crypto/rand"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"runtime/debug"
+	"time"
+
+	"gen2d/internal/logger"
+
+	"github.com/gin-gonic/gin"
+)
+
+// Logger 返回 HTTP 请求日志中间件。
+// 为每个请求生成 request_id，记录 method、path、status、latency、client_ip。
+func Logger() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		start := time.Now()
+		requestID := generateRequestID()
+
+		c.Set("request_id", requestID)
+		ctx, _ := logger.WithRequestID(c.Request.Context(), requestID)
+		c.Request = c.Request.WithContext(ctx)
+
+		c.Header("X-Request-ID", requestID)
+
+		c.Next()
+
+		latency := time.Since(start)
+		status := c.Writer.Status()
+
+		l := logger.FromCtx(ctx)
+		attrs := []slog.Attr{
+			slog.String("method", c.Request.Method),
+			slog.String("path", c.Request.URL.Path),
+			slog.Int("status", status),
+			slog.Duration("latency", latency),
+			slog.String("client_ip", c.ClientIP()),
+		}
+
+		if status >= 500 {
+			l.LogAttrs(ctx, slog.LevelError, "request completed", attrs...)
+		} else if status >= 400 {
+			l.LogAttrs(ctx, slog.LevelWarn, "request completed", attrs...)
+		} else {
+			l.LogAttrs(ctx, slog.LevelInfo, "request completed", attrs...)
+		}
+	}
+}
+
+// Recovery 返回自定义 panic 恢复中间件。
+// panic 时记录完整的 request 上下文和堆栈，返回 500。
+func Recovery() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		defer func() {
+			if r := recover(); r != nil {
+				ctx := c.Request.Context()
+				l := logger.FromCtx(ctx)
+				l.Error("panic recovered",
+					"error", fmt.Sprintf("%v", r),
+					"method", c.Request.Method,
+					"path", c.Request.URL.Path,
+					"request_id", c.GetString("request_id"),
+					"stack", string(debug.Stack()),
+				)
+
+				c.AbortWithStatusJSON(http.StatusInternalServerError,
+					gin.H{"code": http.StatusInternalServerError, "message": "服务器内部错误"})
+			}
+		}()
+		c.Next()
+	}
+}
+
+func generateRequestID() string {
+	b := make([]byte, 8)
+	_, _ = rand.Read(b)
+	return fmt.Sprintf("%d-%x", time.Now().UnixMilli(), b)
+}