feat: add a durable transcode session store

Transcode state lives only in the process that started ffmpeg, so a pod restart
drops every in-flight HLS stream with no way for a peer to pick it up.

- add ITranscodeSessionStore plus the TranscodeSession and LiveStreamSession records
- add RedisTranscodeSessionStore with TTL leases and an atomic Lua takeover script
- add NullTranscodeSessionStore for single-instance deployments
- pick the store from Jellyfin:TranscodeStore:RedisConnectionString at startup

MediaBrowser.Controller/MediaEncoding/TranscodeSession.cs

@@ -0,0 +1,49 @@
+using System;
+
+namespace MediaBrowser.Controller.MediaEncoding;
+
+/// <summary>
+/// Represents a durable record of an HLS transcoding session for HA pod recovery.
+/// </summary>
+public sealed class TranscodeSession
+{
+    /// <summary>
+    /// Gets or sets the unique play session identifier.
+    /// </summary>
+    public string PlaySessionId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the name of the pod that currently owns this session's lease.
+    /// </summary>
+    public string OwnerPod { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the UTC time at which the owning pod's lease expires.
+    /// </summary>
+    public DateTime LeaseExpiresUtc { get; set; }
+
+    /// <summary>
+    /// Gets or sets the absolute path to the HLS manifest (.m3u8) file on shared storage.
+    /// </summary>
+    public string ManifestPath { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the path prefix for transcoded segment files on shared storage.
+    /// </summary>
+    public string SegmentPathPrefix { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the media source identifier associated with this session.
+    /// </summary>
+    public string MediaSourceId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the zero-based index of the last segment that was fully written to durable storage.
+    /// </summary>
+    public int LastCompletedSegmentIndex { get; set; }
+
+    /// <summary>
+    /// Gets or sets the last durable playback offset in ticks, used to resume playback after failover.
+    /// </summary>
+    public long LastDurablePlaybackOffset { get; set; }
+}