feat: 添加 HTTP 请求日志中间件（Phase 4）

实现内容：
- 创建 trace/gin_logger.go，实现 GinLogger 和 GinRecovery 中间件
- GinLogger 自动记录所有 HTTP 请求的 method/path/status/latency/client_ip
- GinRecovery 使用 zap 记录 panic 恢复信息，替代 gin.Recovery()
- 修改 main.go 注册三层中间件：TraceMiddleware -> GinLogger -> GinRecovery
- 所有日志自动附加 trace_id 和 request_id 字段

日志示例：
{
  "level": "info",
  "ts": "2026-06-21T22:26:07.610+0800",
  "msg": "request completed",
  "trace_id": "01KVN964KSY6BFDKB9S932NXB8",
  "request_id": "01KVN964KSY6BFDKB9S932NXB8",
  "method": "GET",
  "path": "/api/health",
  "status": 200,
  "latency_ms": 0,
  "client_ip": "::1"
}

测试：已验证健康检查接口日志正常输出

backend/cmd/server/main.go

@@ -23,6 +23,7 @@ import (
 	"github.com/hhs/camtalk/internal/ratelimit"
 	"github.com/hhs/camtalk/internal/session"
 	"github.com/hhs/camtalk/internal/store"
+	"github.com/hhs/camtalk/internal/trace"
 	"github.com/hhs/camtalk/internal/ws"
 	migrations "github.com/hhs/camtalk/migrations"
 )
@@ -226,7 +227,9 @@ func main() {
 	}
 
 	r := gin.New()
-	r.Use(gin.Recovery())
+	r.Use(trace.TraceMiddleware())    // 第一层：生成 trace ID
+	r.Use(trace.GinLogger())          // 第二层：记录请求
+	r.Use(trace.GinRecovery())        // 第三层：panic 恢复
 
 	// REST API
 	apiGroup := r.Group("/api")

backend/internal/trace/gin_logger.go

@@ -0,0 +1,63 @@
+package trace
+
+import (
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+// GinLogger 记录每个 HTTP 请求的 method/path/status/latency
+func GinLogger() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		start := time.Now()
+		path := c.Request.URL.Path
+		query := c.Request.URL.RawQuery
+
+		c.Next()
+
+		latency := time.Since(start).Milliseconds()
+		status := c.Writer.Status()
+		log := FromContext(c.Request.Context())
+
+		fields := []interface{}{
+			"method", c.Request.Method,
+			"path", path,
+			"status", status,
+			"latency_ms", latency,
+			"client_ip", c.ClientIP(),
+		}
+		if query != "" {
+			fields = append(fields, "query", query)
+		}
+		if errStr := c.Errors.String(); errStr != "" {
+			fields = append(fields, "errors", errStr)
+		}
+
+		switch {
+		case status >= 500:
+			log.Errorw("request completed", fields...)
+		case status >= 400:
+			log.Warnw("request completed", fields...)
+		default:
+			log.Infow("request completed", fields...)
+		}
+	}
+}
+
+// GinRecovery 自定义 panic 恢复中间件，使用 zap 记录
+func GinRecovery() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		defer func() {
+			if err := recover(); err != nil {
+				log := FromContext(c.Request.Context())
+				log.Errorw("panic recovered",
+					"error", err,
+					"path", c.Request.URL.Path,
+					"method", c.Request.Method,
+					"client_ip", c.ClientIP())
+				c.AbortWithStatus(500)
+			}
+		}()
+		c.Next()
+	}
+}