feat(plex): every HLS session reap logs its rule and idle time

backend/app/plex/stream.py

@@ -1185,6 +1185,7 @@ def reap_idle(finished_idle_s: float = _FINISHED_IDLE_S) -> int:
     one a whole segmented copy of the film on a volume shared with the download center."""
     now = time.time()
     doomed: list[tuple[HlsSession, Path | None]] = []
+    reaped: list[tuple[str, str, int]] = []
     with _lock:
         for reg, s in list(_sessions.items()):
             done = s.proc.poll() is not None
@@ -1192,6 +1193,13 @@ def reap_idle(finished_idle_s: float = _FINISHED_IDLE_S) -> int:
             if idle > _SESSION_IDLE_S or (done and idle > finished_idle_s):
                 _sessions.pop(reg, None)
                 doomed.append(_detach(s))
+                reaped.append((s.key, "finished_idle" if done else "idle", int(idle)))
+    # The viewer of a session wrongly taken sees only a 404 later, so this line is the one record of why.
+    for key, reason, idle_s in reaped:
+        log.info(
+            "an idle session was reaped",
+            extra=logs.event("plex.stream.session_reaped", key=key, reason=reason, idle_s=idle_s),
+        )
     _finish(doomed)
     return len(doomed)