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/api/v1/streaming.py

@@ -10,6 +10,7 @@ import re
 import uuid
 from typing import Annotated
 
+import anyio
 from fastapi import APIRouter, Header, Query, Response
 from fastapi.responses import FileResponse, StreamingResponse
 
@@ -78,7 +79,7 @@ async def stream_hls_playlist(
         await enqueue_transcode_quiet(track_id, hls=True)
         raise NotFoundError("HLS rendition is being prepared; retry shortly.")
 
-    body = path.read_text()
+    body = await anyio.to_thread.run_sync(path.read_text)
     if token:
         body = _SEGMENT_LINE_RE.sub(rf"\1?token={token}", body)
     return Response(body, media_type=_HLS_PLAYLIST_TYPE)

app/api/v1/tracks.py

@@ -3,6 +3,7 @@
 import uuid
 from typing import Annotated
 
+import anyio
 from fastapi import APIRouter, Query, Response
 from fastapi.responses import StreamingResponse
 
@@ -33,7 +34,8 @@ from app.api.schemas.track import (
     TrackUpdate,
 )
 from app.api.schemas.transcode import OptimizeEnqueuedOut
-from app.application.transcode_service import bitrate_for_quality
+from app.application.transcode_service import bitrate_for_quality, remove_track_cache
+from app.core.config import get_settings
 from app.domain.entities.album import Album
 from app.domain.entities.track import Artist, Track
 from app.domain.errors import NotFoundError, ValidationError
@@ -219,6 +221,10 @@ async def delete_track(
     await track_repo.delete(track_id)
     if track.storage_uri is not None:
         await storage.delete(track.storage_uri)
+    # Drop any cached transcode renditions (Opus + HLS) so they don't dangle.
+    await anyio.to_thread.run_sync(
+        remove_track_cache, get_settings().transcode_cache_path, track_id
+    )
     return Response(status_code=204)