fix(plex): a lost HLS session is reclaimed at the first answer

The route answers 410 for a session it no longer has and keeps 404 for a segment not there yet.
hls.js retries even a 4xx on a stream with one video variant, so the player waited about half a
minute before asking again, and minutes in a throttled page.

backend/app/plex/stream.py

@@ -1184,7 +1184,7 @@ def reap_idle(finished_idle_s: float = _FINISHED_IDLE_S) -> int:
     session gone that way, and one that lived on the same cadence because its phase was luckier).
     So the rule is sized to the throttled beat (`_THROTTLED_HEARTBEAT_S`). The heartbeat
     stops once the item is finished or before a position is known (`usePlexProgress.flush`) — both
-    states where reaping is correct, and where the player's 404 → reclaim recovers a viewer who
+    states where reaping is correct, and where the player's 410 → reclaim recovers a viewer who
     scrubs back in anyway. Do not restate this as "whether or not it is playing":
     a review found that claim in two comments and not in the code. An earlier blanket
     `reap_abandoned` bought the same safety by keeping every finished session for ten minutes — each