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/tests/test_plex_hls_file_route.py

@@ -88,10 +88,10 @@ def test_another_viewers_session_of_the_same_item_is_not_this_viewers(client, tm
     (d / "index.m3u8").write_bytes(b"#EXTM3U")
     sessions[(7, "m1")] = _Session("sid-a", d)
     r = client.get("/api/plex/stream/m1/hls/sid-a/index.m3u8")
-    assert r.status_code == 404, r.text
+    assert r.status_code == 410, r.text
 
 
-def test_a_segment_of_a_replaced_session_is_a_404_not_the_new_sessions_bytes(
+def test_a_segment_of_a_replaced_session_is_gone_not_the_new_sessions_bytes(
     client, tmp_path, sessions
 ):
     """Viewer A is at 0:00; viewer B seeks to 25:00 and their session takes the registry slot. A's
@@ -102,15 +102,15 @@ def test_a_segment_of_a_replaced_session_is_a_404_not_the_new_sessions_bytes(
     (theirs / "seg_12.ts").write_bytes(b"theirs")
     sessions[(VIEWER, "m1")] = _Session("sid-b", theirs)
     r = client.get("/api/plex/stream/m1/hls/sid-a/seg_12.ts")
-    assert r.status_code == 404, r.text
+    assert r.status_code == 410, r.text
     assert b"theirs" not in r.content
 
 
-def test_a_playlist_with_no_live_session_is_a_404_not_a_silent_restart_at_zero(
+def test_a_playlist_with_no_live_session_is_gone_not_a_silent_restart_at_zero(
     client, monkeypatch, sessions
 ):
     """The entry playlist used to auto-start a session when none was registered — always at 0.0,
-    silently throwing away the viewer's position. A 404 sends the player through its own reclaim,
+    silently throwing away the viewer's position. A 410 sends the player through its own reclaim,
     which restarts at the playhead."""
     started: list[float] = []
     monkeypatch.setattr(
@@ -119,7 +119,7 @@ def test_a_playlist_with_no_live_session_is_a_404_not_a_silent_restart_at_zero(
         lambda *a, **k: started.append(a[3] if len(a) > 3 else 0.0),
     )
     r = client.get("/api/plex/stream/m1/hls/sid-gone/index.m3u8")
-    assert r.status_code == 404, r.text
+    assert r.status_code == 410, r.text
     assert started == []
 
 
@@ -235,12 +235,12 @@ def test_an_absurd_segment_index_is_a_404_not_a_500(client, tmp_path, sessions,
     assert r.status_code == 404, r.text
 
 
-def test_a_session_torn_down_during_the_read_is_a_404_not_a_500(
+def test_a_session_torn_down_during_the_read_is_gone_not_a_500(
     client, monkeypatch, tmp_path, sessions
 ):
     """Teardown moves the whole directory aside and nothing holds `_lock` across the wait and the read,
     so the file can vanish in between — on every seek-restart, since `loadSession` awaits the new
-    session before destroying the old hls instance. The player's recovery is keyed on the 404
+    session before destroying the old hls instance. The player's recovery is keyed on the
     STATUS, so a 500 here skipped the reclaim and ended playback with "couldn't play"."""
     d = tmp_path / "m1_0_0_+0.00"
     d.mkdir()
@@ -249,7 +249,7 @@ def test_a_session_torn_down_during_the_read_is_a_404_not_a_500(
     # The file existed when the wait checked, and is gone by the time the route reads it.
     monkeypatch.setattr(plex_stream, "wait_for", lambda *a, **k: True)
     r = client.get("/api/plex/stream/m1/hls/sid-a/seg_12.ts")
-    assert r.status_code == 404, r.text
+    assert r.status_code == 410, r.text
 
 
 def test_a_dead_producer_is_not_waited_out(client, tmp_path, sessions):