fix(export): explicit Content-Length on the download responses

Both hops (backend attachment + gateway /dl forward) wrote the body
through net/http's buffered writer, so anything over the write buffer
went out Transfer-Encoding: chunked. Android's system DownloadManager —
the executor behind VKWebAppDownloadFile — hangs indefinitely on a
download of unknown length (observed on-device: VK Android stuck in
'downloading'); Telegram's downloader tolerates chunked, which is why
only VK broke. The artifact is fully buffered anyway, so the length is
known — set it on both responses.

gateway/internal/connectsrv/server.go

@@ -14,6 +14,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	"time"
 
@@ -465,6 +466,9 @@ func (s *Server) exportDownloadHandler() http.Handler {
 		}
 		w.Header().Set("X-Content-Type-Options", "nosniff")
 		w.Header().Set("Cache-Control", "private, max-age=0")
+		// Explicit length, or the body goes out chunked and Android's system
+		// DownloadManager (the VKWebAppDownloadFile executor) hangs on it.
+		w.Header().Set("Content-Length", strconv.Itoa(len(data)))
 		_, _ = w.Write(data)
 	})
 }