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/LiveStreamSession.cs

@@ -0,0 +1,36 @@
+using System;
+
+namespace MediaBrowser.Controller.MediaEncoding;
+
+/// <summary>
+/// Represents a durable record of an open live stream session, enabling HA pod recovery
+/// when the owning pod crashes or is evicted.
+/// </summary>
+public sealed class LiveStreamSession
+{
+    /// <summary>
+    /// Gets or sets the live stream identifier (e.g. a TV tuner channel token).
+    /// </summary>
+    public string LiveStreamId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the session identifier of the client that opened this live stream.
+    /// </summary>
+    public string SessionId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the play session identifier associated with this live stream,
+    /// or an empty string when the client did not supply one.
+    /// </summary>
+    public string PlaySessionId { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the name of the pod that currently holds this live stream open.
+    /// </summary>
+    public string OwnerPod { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the UTC time at which this record was created.
+    /// </summary>
+    public DateTime OpenedAtUtc { get; set; }
+}