Add lease-aware cleanup to DeleteTranscodeFileTask (#25)

* Initial plan

* Add GetActiveSessionsAsync to ITranscodeSessionStore and update DeleteTranscodeFileTask for lease-aware cleanup

Co-authored-by: ZoltyMat <177592743+ZoltyMat@users.noreply.github.com>

* Fix Redis exception propagation in GetActiveSessionsAsync for safe abort behavior

Co-authored-by: ZoltyMat <177592743+ZoltyMat@users.noreply.github.com>

* fix: use KeysAsync to resolve CA1849 analyzer violation

Replace synchronous IServer.Keys() with async IServer.KeysAsync()
using await foreach to satisfy CA1849 (TreatWarningsAsErrors).

CA1849: 'IServer.Keys()' synchronously blocks.
Await 'IServer.KeysAsync()' instead.

Line 161 in RedisTranscodeSessionStore.GetActiveSessionsAsync.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ZoltyMat <177592743+ZoltyMat@users.noreply.github.com>
Co-authored-by: mat <mstrommen@gmail.com>

MediaBrowser.Controller/MediaEncoding/NullTranscodeSessionStore.cs

@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -28,4 +30,8 @@ public sealed class NullTranscodeSessionStore : ITranscodeSessionStore
     /// <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>());
 }