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/main.py

@@ -27,6 +27,7 @@ from starlette.middleware.sessions import SessionMiddleware
 from app import auth, state
 from app.config import settings
 from app.db import SessionLocal
+from app.plex import stream as plex_stream
 from app.routes import (
     admin,
     audit as audit_routes,
@@ -67,6 +68,11 @@ async def lifespan(app: FastAPI):
                 "    Open the install wizard (internal access only):\n    %s",
                 url,
             )
+    # No HLS session survives a restart (they live in this process), so anything still on disk is
+    # a crashed run's segments — sweep them or they accumulate forever.
+    swept = plex_stream.sweep_orphans()
+    if swept:
+        log.info("swept %d orphaned HLS segment directories", swept)
     start_scheduler()
     try:
         yield