nixos/pinnwand: improve settings behaviour
nixos/pinnwand: improve settings behaviour

Individual settings would previously overwrite the whole config, but
now individual values can be overwritten.

Fix missing slash to make the database path an absolute path per
https://docs.sqlalchemy.org/en/14/core/engines.html#sqlite.

Drop preferred_lexers, it's not set to anything meaningful anyway.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs