fix(plex): a seek during a quiet reclaim is no longer undone

A quiet request leaves `live` alone, so a seek inside the old session's
range went native and the answer then jumped back to where it asked from.
`seek` now also goes by `pending`; the server-away retry had the same hole.

Comments that still named 404 for a lost session say 410, and the hls.js
retry comment now tells a finished playlist from a growing one.

backend/app/plex/stream.py

@@ -125,7 +125,7 @@ class HlsSession:
         # replaces the viewer's registered session for the item — and the old player's next request
         # for `seg_12.ts`, resolved by (viewer, rating_key) alone, was answered 200 with the NEW session's twelfth segment, produced from a different offset
         # under `-copyts`. Wrong bytes, not an error, so the client's reclaim never fired on it.
-        # A request naming a session that is no longer registered is a 404,
+        # A request naming a session that is no longer registered is a 410,
         # which the player recovers from by restarting at the playhead.
         self.id = secrets.token_urlsafe(9)
         self.key = key  # the item's rating_key (what the logs name)
@@ -729,7 +729,7 @@ def _enforce_caps(
       whole segmented copy of the film under `_HLS_ROOT`, written faster than realtime. Counted only
       by the transcode cap, N distinct items were N ffmpegs and N film-sized scratch directories.
 
-    Somebody's playback stops at each drop. The route turns the missing session into a retryable 404
+    Somebody's playback stops at each drop. The route turns the missing session into a 410
     and the player restarts at the playhead, so from the server nothing failed — and "my film keeps
     rebuffering" has no other explanation anywhere, hence the line."""
 
@@ -918,7 +918,7 @@ def start_session(
             # stale and stops the session it names (`stop_session`), which stopped the playback that won.
             # Never None either: the POST /session route turns a None return into a "Media file not found
             # on disk" 404 — dishonest for a file that exists. A dead session instead degrades to a
-            # retryable 404 ("No active playback session" / "Not available yet") the client re-requests.
+            # 410 ("session_gone") on its URLs, and the client asks again.
             return s
         s = respawned
         ready = wait_for(_seg(0), timeout=25.0, proc=s.proc)
@@ -1200,7 +1200,7 @@ def reap_idle(finished_idle_s: float = _FINISHED_IDLE_S) -> int:
                 _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.
+    # The viewer of a session wrongly taken sees only a 410 later, so this line is the one record of why.
     for key, reason, idle_s in reaped:
         log.info(
             "an idle session was reaped",