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.

Jellyfin.Server.Implementations/Clustering/IFileSystemChangeProcessor.cs

@@ -0,0 +1,39 @@
+// <copyright file="IFileSystemChangeProcessor.cs" company="PlaceholderCompany">
+// Copyright (c) PlaceholderCompany. All rights reserved.
+// </copyright>
+
+namespace Jellyfin.Server.Implementations.Clustering
+{
+    using System.Threading;
+    using System.Threading.Tasks;
+
+    /// <summary>
+    /// Service that processes file system changes from the database (primary instance only).
+    /// </summary>
+    public interface IFileSystemChangeProcessor
+    {
+        /// <summary>
+        /// Starts the file system change processor.
+        /// </summary>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+        Task StartAsync(CancellationToken cancellationToken = default);
+
+        /// <summary>
+        /// Stops the file system change processor.
+        /// </summary>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+        Task StopAsync(CancellationToken cancellationToken = default);
+
+        /// <summary>
+        /// Records a file system change to the database.
+        /// </summary>
+        /// <param name="path">The path that changed.</param>
+        /// <param name="changeType">The type of change (Created, Modified, Deleted, Renamed).</param>
+        /// <param name="oldPath">The old path for rename operations.</param>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+        Task RecordChangeAsync(string path, string changeType, string? oldPath = null, CancellationToken cancellationToken = default);
+    }
+}