feat(playback): pre-transcode HEVC videos in background; veryfast preset

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.

backend/app/tasks/celery.py

@@ -18,6 +18,7 @@ celery_app = Celery(
     include=[
         'app.tasks.scan',
         'app.tasks.thumbs',
+        'app.tasks.video',
         'app.tasks.vision',
         'app.services.metadata',   # extract_metadata lives here
     ]
@@ -52,6 +53,10 @@ celery_app.conf.update(
         'scan_folder': {'queue': 'low'},
         'scan_all_source_roots': {'queue': 'low'},
         'backfill_gps': {'queue': 'low'},
+        # Pre-transcode HEVC videos in the background so /playback is a
+        # cache hit on first user click. CPU-heavy but tolerant of the
+        # low-priority queue (it doesn't block any user-facing flow).
+        'pretranscode_video': {'queue': 'low'},
         # `watch_folders` is retired (file events come from NC webhooks)
         # but the task definition still exists as a no-op shim for any
         # in-flight apply_async. Route it to the default queue so the