perf: 静态文件完全绕过所有中间件

- 使用 gin.New() 替代 gin.Default()，避免默认中间件
- 添加 skipUploads 包装器，所有中间件跳过 /uploads/ 路径
- 静态文件路由注册在最前面，零中间件开销
- 移除 gzipMiddleware 中重复的跳过逻辑

Co-Authored-By: Claude <noreply@anthropic.com>

backend/cmd/main.go

@@ -225,23 +225,19 @@ func startServer() {
 		gin.SetMode(gin.ReleaseMode)
 	}
 
-	r := gin.Default()
+	// 使用 gin.New() 而不是 gin.Default()，避免默认中间件
+	r := gin.New()
 	r.MaxMultipartMemory = 32 << 20
 
-	r.Use(middleware.Logger())
-	r.Use(middleware.Recovery())
-	r.Use(middleware.Cors())
-	r.Use(gzipMiddleware())
-
-	r.Use(func(c *gin.Context) {
-		if strings.HasPrefix(c.Request.URL.Path, "/uploads/") {
-			c.Header("Cache-Control", "public, max-age=31536000, immutable")
-		}
-		c.Next()
-	})
-
-	// 优化的静态文件服务，支持 Range 请求和断点续传
+	// 静态文件路由 - 注册在最前面，不经过任何中间件
 	r.GET("/uploads/*filepath", handleOptimizedStaticFile)
+
+	// 以下中间件只对非 uploads 路径生效
+	r.Use(skipUploads(middleware.Logger()))
+	r.Use(skipUploads(middleware.Recovery()))
+	r.Use(skipUploads(middleware.Cors()))
+	r.Use(skipUploads(gzipMiddleware()))
+
 	router.SetupRoutes(r)
 	setupEmbeddedFrontend(r)
 
@@ -258,6 +254,17 @@ func startServer() {
 	}
 }
 
+// skipUploads 包装中间件，跳过 /uploads/ 路径
+func skipUploads(handler gin.HandlerFunc) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if strings.HasPrefix(c.Request.URL.Path, "/uploads/") {
+			c.Next()
+			return
+		}
+		handler(c)
+	}
+}
+
 func setupEmbeddedFrontend(r *gin.Engine) {
 	distFS, err := fs.Sub(embeddedFiles, "embedded/dist")
 	if err != nil {
@@ -386,12 +393,6 @@ func isCompressibleContentType(contentType string) bool {
 
 func gzipMiddleware() gin.HandlerFunc {
 	return func(c *gin.Context) {
-		// 静态文件下载路径跳过 gzip 处理，避免影响下载性能
-		if strings.HasPrefix(c.Request.URL.Path, "/uploads/") {
-			c.Next()
-			return
-		}
-
 		if !strings.Contains(c.GetHeader("Accept-Encoding"), "gzip") {
 			c.Next()
 			return