iPhone: stop rate-slewing the video for sync; add playback telemetry beacon

server/server.js

@@ -1599,6 +1599,10 @@ app.post('/api/media/:id/redownload', async (c) => {
 // Saves fetch the copy in byte ranges (docs/resumable-downloads-plan.md):
 // the ETag pins the generation, so a resumed save never mixes two copies.
 const prepareSkips = new Map(); // videoId → { reason, at } for /prepare
+app.post('/api/diag', async (c) => {
+  try { const t = (await c.req.text()).slice(0, 20000); console.log('[diag]', t); } catch {}
+  return c.body(null, 204);
+});
 registerDownloadRetryRoute(app, {
   media, priority: HIGH, clearHints: id => { prepareSkips.delete(id); streamCache.delete(id); },
 });