fix(downloads): R5 S1 — downloads robustness

- pick the produced file safely: Path("") is Path("."), which passes .exists(),
  so a missing yt-dlp filepath skipped the staging fallback and crashed in
  with_name ("PosixPath('.') has an empty name") on non-YouTube sources
- unwrap a playlist/multi_video extraction to the entry that carries the media
- run_ffmpeg: drain stderr while it runs (a full 64KB pipe blocked ffmpeg
  forever and hung the worker thread), 1s cancel polling, stall watchdog and a
  bounded wait on exit
- escape apostrophes/backslashes in the ffmpeg concat list (pre-0.51.0 names)
- sweep orphaned HLS segment dirs at startup (nothing else ever reaped them)

backend/app/plex/stream.py

@@ -254,6 +254,27 @@ def wait_for(path: Path, timeout: float = 20.0) -> bool:
     return path.exists()
 
 
+def sweep_orphans() -> int:
+    """Delete segment directories no live session owns.
+
+    Sessions live only in THIS process, so at startup every directory under `_HLS_ROOT` is a
+    leftover from a crashed or killed run — and nothing else ever reaps them (the download GC
+    deliberately skips `.plex-hls` as a system tree), so a few restarts mid-playback leave
+    gigabytes of `.ts` segments behind forever."""
+    with _lock:
+        live = {s.dir for s in _sessions.values()}
+    dropped = 0
+    try:
+        entries = list(_HLS_ROOT.iterdir())
+    except OSError:  # the root doesn't exist yet — nothing to sweep
+        return 0
+    for p in entries:
+        if p.is_dir() and p not in live:
+            shutil.rmtree(p, ignore_errors=True)
+            dropped += 1
+    return dropped
+
+
 def reap_idle() -> int:
     now = time.time()
     dropped = 0