feat(db): add opt-in PostgreSQL database provider

Multi-replica deployments cannot share a SQLite file, so the database has to
move to a server engine before the rest of the HA work is usable.

- add Jellyfin.Database.Providers.PostgreSQL with an EF Core Npgsql provider
- register the provider and a pooled NpgsqlDataSource when DatabaseType is Jellyfin-PostgreSQL
- accept postgresql:// URIs and POSTGRES_CONNECTION_STRING alongside CustomProviderOptions
- add container-backed provider, CRUD, concurrency and migration tests

Jellyfin.Server/Program.cs

@@ -24,6 +24,7 @@ using Jellyfin.Server.ServerSetupApp;
 using MediaBrowser.Common.Configuration;
 using MediaBrowser.Common.Net;
 using MediaBrowser.Controller;
+using MediaBrowser.Controller.Configuration;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
@@ -304,6 +305,10 @@ namespace Jellyfin.Server
                 .AddJellyfinDbContext(startupConfigurationManager, startupConfig)
                 .AddSingleton<IApplicationPaths>(appPaths)
                 .AddSingleton<ServerApplicationPaths>(appPaths)
+                // Required by the NpgsqlDataSource factory in AddJellyfinDbContext when
+                // DatabaseType=Jellyfin-PostgreSQL — the factory resolves this from DI
+                // to read CustomProviderOptions and pool settings.
+                .AddSingleton<IServerConfigurationManager>(startupConfigurationManager)
                 .RegisterStartupLogger();
 
             var startupService = migrationStartupServiceProvider.BuildServiceProvider();