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/IPrimaryElectionService.cs

@@ -0,0 +1,58 @@
+// <copyright file="IPrimaryElectionService.cs" company="PlaceholderCompany">
+// Copyright (c) PlaceholderCompany. All rights reserved.
+// </copyright>
+
+namespace Jellyfin.Server.Implementations.Clustering
+{
+    using System;
+    using System.Threading;
+    using System.Threading.Tasks;
+
+    /// <summary>
+    /// Service for managing primary instance election and coordination.
+    /// </summary>
+    public interface IPrimaryElectionService
+    {
+        /// <summary>
+        /// Event raised when the primary instance changes.
+        /// </summary>
+        event EventHandler<PrimaryInstanceChangedEventArgs>? PrimaryInstanceChanged;
+
+        /// <summary>
+        /// Gets a value indicating whether the current instance is the primary instance.
+        /// </summary>
+        bool IsPrimary { get; }
+
+        /// <summary>
+        /// Gets the ID of the current primary instance, if any.
+        /// </summary>
+        Guid? PrimaryInstanceId { get; }
+
+        /// <summary>
+        /// Starts the primary election service and begins monitoring.
+        /// </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 primary election service.
+        /// </summary>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>A <see cref="Task"/> representing the asynchronous operation.</returns>
+        Task StopAsync(CancellationToken cancellationToken = default);
+
+        /// <summary>
+        /// Runs the primary election algorithm.
+        /// </summary>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        /// <returns>The ID of the elected primary instance.</returns>
+        Task<Guid?> ElectPrimaryAsync(CancellationToken cancellationToken = default);
+
+        /// <summary>
+        /// Checks if this instance should execute scheduled tasks.
+        /// </summary>
+        /// <returns>True if this instance should execute scheduled tasks, false otherwise.</returns>
+        bool ShouldExecuteScheduledTasks();
+    }
+}