Complete multi-instance support: Phases 3–6 & deployment

- Implements Phases 3–6: session isolation, cache coordination, primary election, and file system monitor coordination for Jellyfin with PostgreSQL.
- Adds new database entities (Instance, DistributedLock, FileSystemChange) and EF model configurations.
- Includes SQL migration scripts and EF migration for all required tables, columns, and helper functions.
- Updates Device entity and JellyfinDbContext for multi-instance tracking.
- Integrates new DI services for instance registry, distributed locks, cache coordinator, and primary election.
- Adds publishing profiles (Win/Linux/FrameworkDependent) and automation script for deployment.
- Extensive documentation for architecture, setup, and publishing.
- All changes are backward compatible and build successfully.

Emby.Server.Implementations/Session/SessionManager.cs

@@ -64,6 +64,7 @@ namespace Emby.Server.Implementations.Session
         private readonly IMediaSourceManager _mediaSourceManager;
         private readonly IServerApplicationHost _appHost;
         private readonly IDeviceManager _deviceManager;
+        private readonly Jellyfin.Server.Implementations.Clustering.IInstanceRegistry _instanceRegistry;
         private readonly CancellationTokenRegistration _shutdownCallback;
         private readonly ConcurrentDictionary<string, SessionInfo> _activeConnections
             = new(StringComparer.OrdinalIgnoreCase);
@@ -93,6 +94,7 @@ namespace Emby.Server.Implementations.Session
         /// <param name="deviceManager">Instance of <see cref="IDeviceManager"/> interface.</param>
         /// <param name="mediaSourceManager">Instance of <see cref="IMediaSourceManager"/> interface.</param>
         /// <param name="hostApplicationLifetime">Instance of <see cref="IHostApplicationLifetime"/> interface.</param>
+        /// <param name="instanceRegistry">Instance registry for multi-instance support (optional for backward compatibility).</param>
         public SessionManager(
             ILogger<SessionManager> logger,
             IEventManager eventManager,
@@ -106,7 +108,8 @@ namespace Emby.Server.Implementations.Session
             IServerApplicationHost appHost,
             IDeviceManager deviceManager,
             IMediaSourceManager mediaSourceManager,
-            IHostApplicationLifetime hostApplicationLifetime)
+            IHostApplicationLifetime hostApplicationLifetime,
+            Jellyfin.Server.Implementations.Clustering.IInstanceRegistry instanceRegistry = null)
         {
             _logger = logger;
             _eventManager = eventManager;
@@ -120,6 +123,7 @@ namespace Emby.Server.Implementations.Session
             _appHost = appHost;
             _deviceManager = deviceManager;
             _mediaSourceManager = mediaSourceManager;
+            _instanceRegistry = instanceRegistry;
             _shutdownCallback = hostApplicationLifetime.ApplicationStopping.Register(OnApplicationStopping);
 
             _deviceManager.DeviceOptionsUpdated += OnDeviceManagerDeviceOptionsUpdated;
@@ -156,10 +160,30 @@ namespace Emby.Server.Implementations.Session
         public event EventHandler<SessionEventArgs> SessionControllerConnected;
 
         /// <summary>
-        /// Gets all connections.
+        /// Gets all connections for the current instance.
         /// </summary>
         /// <value>All connections.</value>
-        public IEnumerable<SessionInfo> Sessions => _activeConnections.Values.OrderByDescending(c => c.LastActivityDate);
+        /// <remarks>
+        /// In multi-instance deployments, only returns sessions owned by the current instance.
+        /// In single-instance deployments (when _instanceRegistry is null), returns all sessions.
+        /// </remarks>
+        public IEnumerable<SessionInfo> Sessions
+        {
+            get
+            {
+                if (_instanceRegistry is null)
+                {
+                    // Single-instance mode: return all sessions
+                    return _activeConnections.Values.OrderByDescending(c => c.LastActivityDate);
+                }
+
+                // Multi-instance mode: only return sessions for this instance
+                var currentInstanceId = _instanceRegistry.CurrentInstanceId;
+                return _activeConnections.Values
+                    .Where(s => s.InstanceId.Equals(currentInstanceId))
+                    .OrderByDescending(c => c.LastActivityDate);
+            }
+        }
 
         private void OnDeviceManagerDeviceOptionsUpdated(object sender, GenericEventArgs<Tuple<string, DeviceOptions>> e)
         {
@@ -556,7 +580,8 @@ namespace Emby.Server.Implementations.Session
                 DeviceId = deviceId,
                 ApplicationVersion = appVersion,
                 Id = key.GetMD5().ToString("N", CultureInfo.InvariantCulture),
-                ServerId = _appHost.SystemId
+                ServerId = _appHost.SystemId,
+                InstanceId = _instanceRegistry?.CurrentInstanceId ?? Guid.Empty
             };
 
             var username = user?.Username;