Long videos blocked /playback for the entire encode duration. The fix
is to populate the cache before the user clicks, not when they click.
Changes:
- Extract ffprobe + ffmpeg helpers to services/video.py so the request
handler and the background task share one sync implementation. The
endpoint wraps calls in asyncio.to_thread; celery just calls them.
- New tasks/video.py with pretranscode_video. Idempotent: skips when
the cache is already current and skips passthrough-safe sources
(h264 in mp4/m4v/webm). 30-min task time limit so the long-tail
files (3GP archive, multi-minute 1080p clips) still complete.
- scan_folder now dispatches pretranscode_video alongside
generate_thumbnails / extract_metadata for any new video row.
- POST /library/maintenance/backfill-video-cache enqueues every
active video so the existing library catches up.
- libx264 preset bumped from fast to veryfast. ~2x throughput on this
CPU-only box, output a few % larger but well within disk budget.
- /playback simplifies to: cache check, passthrough if h264 in
web-safe container, else sync transcode (still there as fallback
for races against the queued task).
Once the backfill task drains, /playback should be near-instant for
every video. Any video added afterwards is pre-transcoded at scan
time, so the user keeps that property going forward.
