Retire cancelled playback sessions and preserve VOD content time

Adapted from MikaelKW/nodecast-tv-plus playback lease work beginning at 43a58c7685f541498663b827c4b9d6b3753e7202. Reject superseded requests, release on cancellation, bound live restart attempts and await FFmpeg close before deleting cache.

Co-authored-by: Mikael <mikaelkwessel@gmail.com>

server/routes/subtitle.js

@@ -66,7 +66,7 @@ router.get('/', (req, res) => {
         // console.error(`[Subtitle FFmpeg] ${data}`);
     });
 
-    req.on('close', () => {
+    res.on('close', () => {
         ffmpeg.kill('SIGKILL');
     });