perf(video): stream a capped, faststart rendition instead of the raw upload

THE ANSWER to "why does an already-downloaded video still buffer?" — it was never
the download, and it was not the server. Probing the real uploads on the box:

  d0e49e58…  1920x1080  19.4 Mbps   75 MB / 31 s
  ad929d0b…  1920x1080  19.0 Mbps   27 MB / 11 s
  9f4e0865…   720x1584   3.6 Mbps   14 MB / 31 s

To play a 19 Mbps file the client has to SUSTAIN a 19 Mbps download for the whole
clip. No mobile link does, so the <video> buffer drains every few seconds: buffers,
plays, buffers, plays. Server-side disk read was instant and load was 1.7 on 20
cores throughout — the bottleneck is the media itself, not the delivery path.

Second, independent defect: phone MP4s store `moov` AFTER `mdat` (verified on two
uploads), so the player must fetch the file's tail before it can start at all.

Fix — keep the original bytes untouched (that is what the download button serves,
full quality) and build <id>.web.mp4 beside it: longest side capped at 1280,
~2.5 Mbps ceiling, +faststart. Measured on the 19 Mbps file:

  27.3 MB @ 19.0 Mbps  ->  2.55 MB @ 1.78 Mbps   (10.7x less bandwidth)
  transcode took 2.4 s for an 11.5 s clip

- server/media.js (new): probe, decide, 2-at-a-time background queue. Already
  light + correctly sized + faststart => no rendition at all. Light but wrong atom
  order => remux -c copy (seconds, no re-encode). Otherwise re-encode. A rendition
  that lands bigger than the original is discarded. MP4 box-walker for the
  faststart test is unit-checked against known fast/slow files, both directions.
- /stream/<id> serves the rendition, falling back to the original while it is still
  transcoding, so a video is never unplayable. /files/<id> is unchanged and still
  serves the pristine original for download.
- Renditions are queued at upload, and backfilled 15 s after boot for the videos
  that predate this. Range serving is now one shared helper for both routes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/routes.js

@@ -1716,6 +1716,9 @@ route('POST', '/api/messages/upload', async (req, res) => {
       try { fs.renameSync(tmp, path.join(UPLOADS_DIR, id)); }
       catch (e) { try { fs.unlinkSync(tmp); } catch (_) {} return finish(500, { error: 'could not store file' }); }
       R.attachments.create({ id, teamId: u.team_id, uploaderId: u.id, name, mime, size: total });
+      // Videos: build the capped/faststart streaming rendition in the background so it is ready before
+      // anyone taps play. Never blocks the upload response, and playback falls back to the original.
+      try { require('./media').ensureWebRendition(id, mime); } catch (e) {}
       finish(200, { id, name, mime, size: total });
     });
   });