fix: index.html禁用缓存，assets启用长期缓存

- index.html: Cache-Control: no-cache，防止浏览器缓存旧版本
- assets: Cache-Control: max-age=31536000，1年强缓存

backend/cmd/main.go

@@ -262,6 +262,7 @@ func setupEmbeddedFrontend(r *gin.Engine) {
 	}
 
 	r.GET("/assets/*filepath", func(c *gin.Context) {
+		c.Header("Cache-Control", "public, max-age=31536000, immutable")
 		c.FileFromFS(c.Request.URL.Path, http.FS(distFS))
 	})
 
@@ -278,6 +279,7 @@ func setupEmbeddedFrontend(r *gin.Engine) {
 			return
 		}
 
+		c.Header("Cache-Control", "no-cache, no-store, must-revalidate")
 		serveIndexHTML(c, string(indexHTML))
 	})
 }