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/routers/library.py

@@ -842,6 +842,34 @@ async def trigger_backfill_phashes(current_user: User = Depends(get_current_user
         return {"status": "error", "message": str(e)}
 
 
+@router.post("/maintenance/backfill-video-cache")
+async def trigger_backfill_video_cache(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Pre-transcode every active video in the library so /playback hits
+    the cache on first user click instead of paying the encode cost
+    inline. Idempotent — pretranscode_video skips photos whose cache is
+    already populated and current. CPU-bound; runs on the low-priority
+    queue so it doesn't fight thumbnails or other user-facing tasks."""
+    from app.tasks.video import pretranscode_video
+    result = await db.execute(
+        select(Photo.id, Photo.filepath).where(
+            Photo.media_type == 'video',
+            Photo.is_discarded.is_(False),
+        )
+    )
+    rows = result.all()
+    queued = 0
+    for photo_id, filepath in rows:
+        try:
+            pretranscode_video.delay(photo_id, filepath)
+            queued += 1
+        except Exception as e:
+            logger.warning(f"failed to queue pretranscode for {photo_id}: {e}")
+    return {"status": "queued", "count": queued}
+
+
 @router.post("/maintenance/start-watcher")
 async def start_file_watcher(current_user: User = Depends(get_current_user)):
     """Start the filesystem watcher. Uses a Redis lock so only one