Add automatic PostgreSQL database creation and migration

- Added Jellyfin.Database.Providers.Postgres project with full EF Core migration support for PostgreSQL.
- Implemented automatic database creation and privilege assignment on startup.
- Generated initial migration and model snapshot for PostgreSQL schema.
- Updated build, test, and dependency files to include PostgreSQL provider and Npgsql packages.
- Added PowerShell script for generating and testing PostgreSQL migrations.
- No changes to application logic outside database provider/migration infrastructure.

Jellyfin.Server.Implementations/Extensions/ServiceCollectionExtensions.cs

@@ -12,6 +12,7 @@ using System.Reflection;
 using Jellyfin.Database.Implementations;
 using Jellyfin.Database.Implementations.DbConfiguration;
 using Jellyfin.Database.Implementations.Locking;
+using Jellyfin.Database.Providers.Postgres;
 using Jellyfin.Database.Providers.Sqlite;
 using MediaBrowser.Common.Configuration;
 using MediaBrowser.Controller.Configuration;
@@ -28,6 +29,9 @@ public static class ServiceCollectionExtensions
     private static IEnumerable<Type> DatabaseProviderTypes()
     {
         yield return typeof(SqliteDatabaseProvider);
+        yield return typeof(PostgresDatabaseProvider);
+        // Add additional built-in providers here:
+        // yield return typeof(MySqlDatabaseProvider);
     }
 
     private static IDictionary<string, JellyfinDbProviderFactory> GetSupportedDbProviders()