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/DistributedLockNames.cs

@@ -0,0 +1,99 @@
+// <copyright file="DistributedLockNames.cs" company="PlaceholderCompany">
+// Copyright (c) PlaceholderCompany. All rights reserved.
+// </copyright>
+
+namespace Jellyfin.Server.Implementations.Clustering
+{
+    using System;
+
+    /// <summary>
+    /// Standard lock names for distributed locking.
+    /// </summary>
+    public static class DistributedLockNames
+    {
+        /// <summary>
+        /// Lock for database migrations.
+        /// </summary>
+        public const string DatabaseMigration = "database:migration";
+
+        /// <summary>
+        /// Lock for primary instance election.
+        /// </summary>
+        public const string PrimaryElection = "instance:primary:election";
+
+        /// <summary>
+        /// Global lock for all library scans.
+        /// </summary>
+        public const string GlobalLibraryScan = "library:scan:global";
+
+        /// <summary>
+        /// Lock for library scanning operations.
+        /// </summary>
+        /// <param name="libraryId">The library ID.</param>
+        /// <returns>The lock name.</returns>
+        public static string LibraryScan(Guid libraryId)
+        {
+            return $"library:scan:{libraryId}";
+        }
+
+        /// <summary>
+        /// Lock for metadata refresh operations.
+        /// </summary>
+        /// <param name="itemId">The item ID.</param>
+        /// <returns>The lock name.</returns>
+        public static string MetadataRefresh(Guid itemId)
+        {
+            return $"metadata:refresh:{itemId}";
+        }
+
+        /// <summary>
+        /// Lock for configuration updates.
+        /// </summary>
+        /// <param name="configType">The configuration type.</param>
+        /// <returns>The lock name.</returns>
+        public static string ConfigurationUpdate(string configType)
+        {
+            return $"config:update:{configType}";
+        }
+
+        /// <summary>
+        /// Lock for scheduled task execution.
+        /// </summary>
+        /// <param name="taskName">The task name.</param>
+        /// <returns>The lock name.</returns>
+        public static string ScheduledTask(string taskName)
+        {
+            return $"task:{taskName}";
+        }
+
+        /// <summary>
+        /// Lock for image processing operations.
+        /// </summary>
+        /// <param name="itemId">The item ID.</param>
+        /// <returns>The lock name.</returns>
+        public static string ImageProcessing(Guid itemId)
+        {
+            return $"image:process:{itemId}";
+        }
+
+        /// <summary>
+        /// Lock for cache clearing operations.
+        /// </summary>
+        /// <param name="cacheType">The cache type.</param>
+        /// <returns>The lock name.</returns>
+        public static string CacheClear(string cacheType)
+        {
+            return $"cache:clear:{cacheType}";
+        }
+
+        /// <summary>
+        /// Lock for plugin operations.
+        /// </summary>
+        /// <param name="pluginId">The plugin ID.</param>
+        /// <returns>The lock name.</returns>
+        public static string PluginOperation(Guid pluginId)
+        {
+            return $"plugin:operation:{pluginId}";
+        }
+    }
+}