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.

frontend/src/modules/plex/PlexPlayer.hls.dom.test.tsx

@@ -134,13 +134,13 @@ const lost404 = {
   fatal: true,
   type: Hls.ErrorTypes.NETWORK_ERROR,
   details: "fragLoadError",
-  response: { code: 404, text: "No active playback session" },
+  response: { code: 404, text: "Not available yet" },
 };
 
-describe("an HLS session the server no longer has", () => {
+describe("an HLS segment that never arrives (a fatal 404)", () => {
   it("is restarted where the viewer is, instead of ending playback", async () => {
-    // The server reaps a session nobody fetches from (a long pause; a deploy). hls.js never retries
-    // a 4xx, so the next segment's 404 used to end playback with "couldn't play".
+    // Once hls.js has given up on it, the 404 used to end playback with "couldn't play". A session
+    // the server no longer has answers 410 instead: `hlsEngine.test.ts` holds that case.
     const session = mount();
     await waitFor(() => expect(hlsInstances).toHaveLength(1));
     act(() => hlsInstances[0]!.emit(Hls.Events.MANIFEST_PARSED));
@@ -155,10 +155,9 @@ describe("an HLS session the server no longer has", () => {
 
   it("gives up and says so when the reclaimed session keeps 404ing", async () => {
     // `parsed` cannot bound this: it lives on ONE hls instance and every restart builds a fresh one,
-    // so a session that parses its manifest and then 404s re-arms the guard every cycle. The server
-    // also answers 404 for "that segment is not there YET" (a transcode falling behind realtime),
-    // which is indistinguishable on the wire — so the player respawned ffmpeg forever behind a
-    // spinner that never became an error.
+    // so a session that parses its manifest and then 404s re-arms the guard every cycle. A transcode
+    // falling behind realtime answers 404 for "that segment is not there YET" every time, so the
+    // player respawned ffmpeg forever behind a spinner that never became an error.
     const session = mount();
     const v = () => document.querySelector("video") as Media;
     // The first session plus the two it is allowed to reclaim; the third 404 has nothing left.