fix(plex): a segment name ffmpeg never spells is refused; teardown no longer holds the registry lock

The artifact guard parsed the sequence with int(), so a zero-padded or non-ASCII-digit name read
as a segment the session had reached and parked the sync route for the full 20 s wait. It now
also requires the canonical spelling segment_name() produces.

Stopping, replacing, culling or reaping a session waited up to 3 s for ffmpeg and deleted a whole
segmented film under _lock, which every segment request and checkpoint takes. Teardown now signals
the process and moves its directory aside under the lock, and waits/deletes after it. A new session
on the same path still waits for the dying ffmpeg first (it writes by path), and the orphan sweep
recognises a moved-aside directory left by a crash.

backend/tests/test_hls_reaper.py

@@ -83,6 +83,7 @@ class _Proc:
 
 class _Session:
     def __init__(self, tmp_path, name: str, finished: bool, idle_s: float):
+        self.key = name
         self.dir = tmp_path / name
         self.dir.mkdir()
         self.proc = _Proc(finished)