fix: POC 扫描接入调用方 context，修复 cachedPocPath 竞争和 ProxyStats data race

- webscan/web_scan.go: WebScan 接受 ctx 参数，替换 context.Background()；
  sync.Once 改为 sync.Mutex 保护 POC 加载，消除 cachedPocPath 并发写竞争
- webtitle.go: ctx 从 Scan 穿透到 identifyFingerprintsMulti → triggerPocScan → WebScan
- webpoc.go: 传递 ctx 到 WebScan
- proxy/types.go: ProxyStats 增加 sync.Mutex
- proxy/manager.go: LastConnectTime/LastError/AverageConnectTime 读写加锁

common/proxy/types.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/tls"
 	"net"
+	"sync"
 	"time"
 )
 
@@ -95,9 +96,10 @@ type ProxyManager interface {
 //
 //nolint:revive // 保持与现有代码的向后兼容性
 type ProxyStats struct {
-	TotalConnections   int64         `json:"total_connections"`
-	ActiveConnections  int64         `json:"active_connections"`
-	FailedConnections  int64         `json:"failed_connections"`
+	TotalConnections   int64  `json:"total_connections"`
+	ActiveConnections  int64  `json:"active_connections"`
+	FailedConnections  int64  `json:"failed_connections"`
+	mu                 sync.Mutex    `json:"-"`
 	AverageConnectTime time.Duration `json:"average_connect_time"`
 	LastConnectTime    time.Time     `json:"last_connect_time"`
 	LastError          string        `json:"last_error,omitempty"`