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/ITranscodeSessionStore.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -59,4 +60,14 @@ public interface ITranscodeSessionStore
     /// <param name="cancellationToken">A cancellation token.</param>
     /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
     Task DeleteAsync(string playSessionId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Returns all currently active transcoding sessions from the store.
+    /// </summary>
+    /// <param name="cancellationToken">A cancellation token.</param>
+    /// <returns>
+    /// An enumerable of <see cref="TranscodeSession"/> objects representing all active sessions.
+    /// Returns an empty enumerable if no sessions are active or if the store cannot be reached.
+    /// </returns>
+    Task<IEnumerable<TranscodeSession>> GetActiveSessionsAsync(CancellationToken cancellationToken = default);
 }