I have already checked through the existing feature requests and found no duplicates

• Yes

Is this a server-specific feature?

Navidrome

What do you want to be added?

I have setup my navidrome server with some custom tags for my songs. I want to use those tags to create some smart playlists.
Currently the smart playlist editor only allows to create rules from a fixed pool of tags defined by NDSongQueryFields.
Having the option to use any tag for smart playlists would be great.

Either adding the option to just type arbitrary tag into the select field and assuming its type is string or giving the option to select what type the tag has (string, number) could be one option to implement it. Or adding a entry in the settings to specify custom tags, which could then also be used in the song table as a column.