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

tests/Jellyfin.MediaEncoding.Tests/Fakes/InMemoryTranscodeSessionStore.cs

@@ -0,0 +1,169 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using MediaBrowser.Controller.MediaEncoding;
+
+namespace Jellyfin.MediaEncoding.Tests.Fakes;
+
+/// <summary>
+/// Thread-safe, in-memory implementation of <see cref="ITranscodeSessionStore"/> for use in unit tests.
+/// </summary>
+public sealed class InMemoryTranscodeSessionStore : ITranscodeSessionStore
+{
+    /// <summary>
+    /// The duration added to <see cref="DateTime.UtcNow"/> when a lease is renewed or first claimed.
+    /// </summary>
+    public static readonly TimeSpan DefaultLeaseDuration = TimeSpan.FromSeconds(30);
+
+    private readonly Dictionary<string, TranscodeSession> _sessions = new(StringComparer.OrdinalIgnoreCase);
+    private readonly Dictionary<string, LiveStreamSession> _liveStreams = new(StringComparer.OrdinalIgnoreCase);
+    private readonly Lock _lock = new();
+
+    /// <inheritdoc />
+    public Task<TranscodeSession?> TryGetAsync(string playSessionId, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            if (_sessions.TryGetValue(playSessionId, out var session) && session.LeaseExpiresUtc > DateTime.UtcNow)
+            {
+                return Task.FromResult<TranscodeSession?>(Clone(session));
+            }
+
+            return Task.FromResult<TranscodeSession?>(null);
+        }
+    }
+
+    /// <inheritdoc />
+    public Task<bool> TryTakeoverAsync(string playSessionId, string claimingPod, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            if (!_sessions.TryGetValue(playSessionId, out var session))
+            {
+                return Task.FromResult(false);
+            }
+
+            if (session.LeaseExpiresUtc > DateTime.UtcNow)
+            {
+                // Another pod's lease is still valid – takeover not permitted.
+                return Task.FromResult(false);
+            }
+
+            // Lease has expired – claim it atomically.
+            session.OwnerPod = claimingPod;
+            session.LeaseExpiresUtc = DateTime.UtcNow.Add(DefaultLeaseDuration);
+            return Task.FromResult(true);
+        }
+    }
+
+    /// <inheritdoc />
+    public Task SetAsync(TranscodeSession session, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            _sessions[session.PlaySessionId] = session;
+        }
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task RenewLeaseAsync(string playSessionId, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            if (_sessions.TryGetValue(playSessionId, out var session))
+            {
+                session.LeaseExpiresUtc = DateTime.UtcNow.Add(DefaultLeaseDuration);
+            }
+        }
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task DeleteAsync(string playSessionId, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            _sessions.Remove(playSessionId);
+        }
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task<IEnumerable<TranscodeSession>> GetActiveSessionsAsync(CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            var sessions = _sessions.Values.Select(Clone).ToList();
+            return Task.FromResult<IEnumerable<TranscodeSession>>(sessions);
+        }
+    }
+
+    /// <inheritdoc />
+    public Task SetLiveStreamAsync(LiveStreamSession session, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            _liveStreams[MakeLiveStreamKey(session.LiveStreamId, session.SessionId)] = session;
+            if (!string.IsNullOrEmpty(session.PlaySessionId))
+            {
+                _liveStreams[MakeLiveStreamKey(session.LiveStreamId, session.PlaySessionId)] = session;
+            }
+        }
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task<LiveStreamSession?> TryGetLiveStreamAsync(string liveStreamId, string sessionIdOrPlaySessionId, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            _liveStreams.TryGetValue(MakeLiveStreamKey(liveStreamId, sessionIdOrPlaySessionId), out var session);
+            return Task.FromResult(session);
+        }
+    }
+
+    /// <inheritdoc />
+    public Task DeleteLiveStreamAsync(string liveStreamId, string sessionIdOrPlaySessionId, CancellationToken cancellationToken = default)
+    {
+        lock (_lock)
+        {
+            if (_liveStreams.TryGetValue(MakeLiveStreamKey(liveStreamId, sessionIdOrPlaySessionId), out var session))
+            {
+                _liveStreams.Remove(MakeLiveStreamKey(liveStreamId, session.SessionId));
+                if (!string.IsNullOrEmpty(session.PlaySessionId))
+                {
+                    _liveStreams.Remove(MakeLiveStreamKey(liveStreamId, session.PlaySessionId));
+                }
+            }
+            else
+            {
+                _liveStreams.Remove(MakeLiveStreamKey(liveStreamId, sessionIdOrPlaySessionId));
+            }
+        }
+
+        return Task.CompletedTask;
+    }
+
+    private static string MakeLiveStreamKey(string liveStreamId, string sessionIdOrPlaySessionId)
+        => liveStreamId + "\x00" + sessionIdOrPlaySessionId;
+
+    private static TranscodeSession Clone(TranscodeSession source)
+        => new TranscodeSession
+        {
+            PlaySessionId = source.PlaySessionId,
+            OwnerPod = source.OwnerPod,
+            LeaseExpiresUtc = source.LeaseExpiresUtc,
+            ManifestPath = source.ManifestPath,
+            SegmentPathPrefix = source.SegmentPathPrefix,
+            MediaSourceId = source.MediaSourceId,
+            LastCompletedSegmentIndex = source.LastCompletedSegmentIndex,
+            LastDurablePlaybackOffset = source.LastDurablePlaybackOffset,
+        };
+}