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/scan.py

@@ -20,6 +20,7 @@ from app.database import AsyncSessionLocal
 from app.models import Photo, Folder, SourceRoot
 from app.config import settings
 from app.tasks.thumbs import generate_thumbnails
+from app.tasks.video import pretranscode_video
 from app.services.metadata import extract_metadata
 from app.services.date_guess import has_date_warning
 
@@ -195,6 +196,7 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                     # Defer task dispatch until AFTER commit so workers don't
                     # query for rows that aren't visible to other sessions yet.
                     pending_dispatch: list[str] = []
+                    pending_video_pretranscode: list[tuple[str, str]] = []
 
                     for filename in batch:
                         filepath = os.path.join(root, filename)
@@ -316,6 +318,13 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                             # below; otherwise the worker can race the writer
                             # and see "Photo not found".
                             pending_dispatch.append(photo.id)
+                            if photo.media_type == 'video':
+                                # Pre-transcode HEVC and other non-web-safe
+                                # videos at scan time so the user doesn't
+                                # pay the encode cost on first <video> click.
+                                pending_video_pretranscode.append(
+                                    (photo.id, filepath)
+                                )
 
                             processed_files += 1
                             progress_set(REDIS_KEY_PROCESSED, processed_files)
@@ -344,6 +353,8 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                     for photo_id in pending_dispatch:
                         generate_thumbnails.delay(photo_id)
                         extract_metadata.delay(photo_id)
+                    for vid_photo_id, vid_path in pending_video_pretranscode:
+                        pretranscode_video.delay(vid_photo_id, vid_path)
 
             # Update folder scan timestamp
             folder.last_scanned = datetime.utcnow()