refactor(transcode): thread fs ops + atomic cache publish

- run blocking Path.exists/read_text off the event loop via anyio.to_thread
- publish HLS renditions by building into a temp dir then atomic rename, so a
  reader never sees a playlist referencing half-written segments
- per-writer temp name for opus so two concurrent jobs can't interleave
- drop a track's cached renditions on delete so they don't dangle

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

app/application/transcode_service.py

@@ -13,9 +13,12 @@ importing the other.
 """
 
 import re
+import shutil
 import uuid
 from pathlib import Path
 
+import anyio
+
 from app.domain.errors import NotFoundError
 from app.domain.ports import TrackRepository
 
@@ -60,6 +63,12 @@ def hls_segment_path(root: Path, track_id: uuid.UUID, name: str) -> Path | None:
     return hls_dir(root, track_id) / name
 
 
+def remove_track_cache(root: Path, track_id: uuid.UUID) -> None:
+    """Delete every cached rendition for a track (Opus + HLS). Best-effort — used
+    when a track is deleted so its transcode cache doesn't dangle forever."""
+    shutil.rmtree(track_cache_dir(root, track_id), ignore_errors=True)
+
+
 class TranscodeService:
     """Request-side cache lookups for transcoded renditions."""
 
@@ -83,14 +92,16 @@ class TranscodeService:
         if bitrate is None:
             return None
         path = opus_path(self._root, track_id, bitrate)
-        return path if path.exists() else None
+        exists = await anyio.to_thread.run_sync(path.exists)
+        return path if exists else None
 
     async def hls_playlist(self, track_id: uuid.UUID) -> Path | None:
         """Cached HLS playlist if generated, else ``None`` (validates the track
         exists so an unknown id 404s rather than silently missing)."""
         await self._require_streamable(track_id)
         path = hls_playlist_path(self._root, track_id)
-        return path if path.exists() else None
+        exists = await anyio.to_thread.run_sync(path.exists)
+        return path if exists else None
 
     def hls_segment(self, track_id: uuid.UUID, name: str) -> Path | None:
         path = hls_segment_path(self._root, track_id, name)