feat(hls): register transcode sessions and renew their lease

A transcode that is not in the store is invisible to the other replicas, so the
HLS entry points have to publish and hold the lease themselves.

- register the play session in the store when ffmpeg starts
- renew the lease on a background loop and delete the session when it ends
- shorten segments and bound the playlist window when resuming a stored session
- add RecoverySegmentLengthSeconds and RecoverySegmentBufferCount encoding options

MediaBrowser.Model/Configuration/EncodingOptions.cs

@@ -25,6 +25,8 @@ public class EncodingOptions
         ThrottleDelaySeconds = 180;
         EnableSegmentDeletion = false;
         SegmentKeepSeconds = 720;
+        RecoverySegmentLengthSeconds = 2;
+        RecoverySegmentBufferCount = 5;
         EncodingThreadCount = -1;
         // This is a DRM device that is almost guaranteed to be there on every intel platform,
         // plus it's the default one in ffmpeg if you don't specify anything
@@ -125,6 +127,20 @@ public class EncodingOptions
     /// </summary>
     public int SegmentKeepSeconds { get; set; }
 
+    /// <summary>
+    /// Gets or sets the HLS segment length in seconds to use when HA recovery mode is active.
+    /// Shorter segments allow a takeover pod to resume playback faster after a peer failure.
+    /// Default is <c>2</c>. Valid range is 1-6.
+    /// </summary>
+    public int RecoverySegmentLengthSeconds { get; set; }
+
+    /// <summary>
+    /// Gets or sets the number of HLS segments to keep on disk when HA recovery mode is active.
+    /// This acts as a rolling buffer that a takeover pod can serve while restarting the transcode.
+    /// Default is <c>5</c>. Valid range is 2-10.
+    /// </summary>
+    public int RecoverySegmentBufferCount { get; set; }
+
     /// <summary>
     /// Gets or sets the hardware acceleration type.
     /// </summary>