Rate-limit save downloads so they no longer starve playback

server/server.js

@@ -43,6 +43,11 @@ const PORT        = parseInt(process.env.PORT || '3000', 10);
 const APP_VERSION = process.env.APP_VERSION || '1.0.0';
 const YTDLP       = process.env.YTDLP_PATH  || 'yt-dlp';
 const FFMPEG      = process.env.FFMPEG_PATH || 'ffmpeg';
+// Cap for server-side SAVE downloads. yt-dlp with -N 4 pinned the homelab's
+// whole downlink (~7.6 MB/s measured), and since /api/play fetches its own
+// googlevideo slices over the same link, one long save starved every
+// concurrent playback (stalled at ~27 s, 11 KB/s). Leave headroom.
+const DOWNLOAD_RATE = process.env.DOWNLOAD_RATE || '3M';
 
 // ----------------------------------------------------------------------------
 // BUILD_TAG — must be DETERMINISTIC across restarts of identical code.
@@ -677,7 +682,7 @@ async function ytdlpDownloadResponse(videoId, fp, formatArgs) {
       `https://www.youtube.com/watch?v=${videoId}`,
       '--no-warnings', '--no-playlist',
       ...formatArgs,
-      '-N', '4',
+      '--limit-rate', DOWNLOAD_RATE,
       '-o', tmp,
     ]);
     size = statSync(tmp).size;
@@ -730,7 +735,7 @@ async function ytdlpEditedDownloadResponse(videoId, fp, keep) {
       '--no-warnings', '--no-playlist',
       '-f', 'bv*[height<=720][ext=mp4]+ba[ext=m4a]/bv*[height<=720]+ba/b[ext=mp4]/b',
       '--merge-output-format', 'mp4',
-      '-N', '4',
+      '--limit-rate', DOWNLOAD_RATE,
       '-o', srcTmp,
     ]);
     // 2) Trim + concat the keep segments into the final custom video.