fix(proxy): prevent use-after-recycle in background cache refresh

Copy the request before spawning the background goroutine. The
fasthttp.RequestCtx is recycled after the handler returns, so passing
it to a goroutine causes data corruption under high concurrency.

The caller now AcquireRequest+CopyTo before go(), and the goroutine
releases it. backgroundRefresh no longer accepts ctx directly.

internal/proxy/cache_handler.go

@@ -55,20 +55,14 @@ func (p *Proxy) writeCachedResponse(ctx *fasthttp.RequestCtx, entry *cache.Proxy
 // 该方法在独立 goroutine 中运行，不阻塞主请求流程。
 //
 // 参数：
-//   - ctx: 原始 FastHTTP 请求上下文（仅用于复制请求信息）
+//   - req: 预复制的请求副本（调用方负责 Acquire/Release）
 //   - target: 要刷新的后端目标
 //   - hashKey: 缓存哈希键
 //   - origKey: 缓存原始键
-func (p *Proxy) backgroundRefresh(ctx *fasthttp.RequestCtx, target *loadbalance.Target, hashKey uint64, origKey string) {
-	// 创建新的请求上下文副本
-	req := fasthttp.AcquireRequest()
+func (p *Proxy) backgroundRefresh(req *fasthttp.Request, target *loadbalance.Target, hashKey uint64, origKey string) {
 	resp := fasthttp.AcquireResponse()
-	defer fasthttp.ReleaseRequest(req)
 	defer fasthttp.ReleaseResponse(resp)
 
-	// 复制原始请求
-	ctx.Request.CopyTo(req)
-
 	// 如果启用 Revalidate，添加条件请求头
 	if p.config.Cache.Revalidate {
 		if entry, ok, _ := p.cache.Get(hashKey, origKey); ok {

internal/proxy/proxy.go

@@ -651,14 +651,17 @@ func (p *Proxy) ServeHTTP(ctx *fasthttp.RequestCtx) {
 						}
 						return
 					}
-					// 过期缓存，尝试后台刷新，同时返回旧数据
-					if !p.config.Cache.BackgroundUpdateDisable {
-						entry.Updating.Store(true)
-						go func() {
-							defer entry.Updating.Store(false)
-							p.backgroundRefresh(ctx, target, hashKey, origKey)
-						}()
-					}
+				// 过期缓存，尝试后台刷新，同时返回旧数据
+				if !p.config.Cache.BackgroundUpdateDisable {
+					entry.Updating.Store(true)
+					reqCopy := fasthttp.AcquireRequest()
+					ctx.Request.CopyTo(reqCopy)
+					go func() {
+						defer entry.Updating.Store(false)
+						defer fasthttp.ReleaseRequest(reqCopy)
+						p.backgroundRefresh(reqCopy, target, hashKey, origKey)
+					}()
+				}
 					upstreamAddr = upstreamCache
 					upstreamStatus = entry.Status