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/NullTranscodeSessionStore.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Controller.MediaEncoding;
+
+/// <summary>
+/// A no-op implementation of <see cref="ITranscodeSessionStore"/> used in single-instance deployments
+/// where durable session tracking across pods is not required.
+/// </summary>
+public sealed class NullTranscodeSessionStore : ITranscodeSessionStore
+{
+    /// <inheritdoc />
+    public Task<TranscodeSession?> TryGetAsync(string playSessionId, CancellationToken cancellationToken = default)
+        => Task.FromResult<TranscodeSession?>(null);
+
+    /// <inheritdoc />
+    public Task<bool> TryTakeoverAsync(string playSessionId, string claimingPod, CancellationToken cancellationToken = default)
+        => Task.FromResult(false);
+
+    /// <inheritdoc />
+    public Task SetAsync(TranscodeSession session, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+
+    /// <inheritdoc />
+    public Task RenewLeaseAsync(string playSessionId, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+
+    /// <inheritdoc />
+    public Task DeleteAsync(string playSessionId, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+
+    /// <inheritdoc />
+    public Task<IEnumerable<TranscodeSession>> GetActiveSessionsAsync(CancellationToken cancellationToken = default)
+        => Task.FromResult<IEnumerable<TranscodeSession>>(Array.Empty<TranscodeSession>());
+
+    /// <inheritdoc />
+    public Task SetLiveStreamAsync(LiveStreamSession session, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+
+    /// <inheritdoc />
+    public Task<LiveStreamSession?> TryGetLiveStreamAsync(string liveStreamId, string sessionIdOrPlaySessionId, CancellationToken cancellationToken = default)
+        => Task.FromResult<LiveStreamSession?>(null);
+
+    /// <inheritdoc />
+    public Task DeleteLiveStreamAsync(string liveStreamId, string sessionIdOrPlaySessionId, CancellationToken cancellationToken = default)
+        => Task.CompletedTask;
+}