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)