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
