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/workers/tasks/transcode_task.py

@@ -7,9 +7,13 @@ enqueues (e.g. several plays before the first finishes) are cheap. The DB sessio
 is released before ffmpeg runs; the track entity is a detached value object.
 """
 
+import shutil
 import uuid
+from pathlib import Path
 from typing import Any
 
+import anyio
+
 from app.application.transcode_service import (
     HLS_BITRATE,
     bitrate_for_quality,
@@ -27,6 +31,15 @@ from app.infrastructure.transcode.ffmpeg import FfmpegTranscoder
 log = get_logger("worker.transcode")
 
 
+def _swap_dir(tmp_dir: Path, final_dir: Path) -> None:
+    """Publish a freshly-built HLS rendition atomically: replace the final dir in
+    one rename so a reader never sees a playlist referencing half-written
+    segments. Any stale partial at the destination is cleared first."""
+    if final_dir.exists():
+        shutil.rmtree(final_dir)
+    tmp_dir.replace(final_dir)
+
+
 async def transcode_track(
     _ctx: dict[str, Any],
     *,
@@ -57,7 +70,12 @@ async def transcode_track(
                 await transcoder.to_opus(src, dest, bitrate_kbps=bitrate)
                 did_opus = True
         if hls and not hls_playlist_path(root, tid).exists():
-            await transcoder.to_hls(src, hls_dir(root, tid), bitrate_kbps=HLS_BITRATE)
+            # Build into a private temp dir, then swap it in atomically — the
+            # cached playlist only becomes visible once every segment is written.
+            final_dir = hls_dir(root, tid)
+            tmp_dir = final_dir.with_name(f"hls.{uuid.uuid4().hex}.tmp")
+            await transcoder.to_hls(src, tmp_dir, bitrate_kbps=HLS_BITRATE)
+            await anyio.to_thread.run_sync(_swap_dir, tmp_dir, final_dir)
             did_hls = True
 
     log.info("transcode_done", track_id=track_id, opus=did_opus, hls=did_hls)