feat: rename or merge an artist folder

Two spellings of one artist is the most common mess a rebuild produces, and fixing it by hand means moving two directories, editing every journal record underneath them and finding every artists.toml entry that named the old name. Missing any one of those leaves the journal describing directories that no longer exist, which library verify then reports as albums that have gone missing.

intake artist rename moves the artist folder in the archive and the library and repoints every journal record and alias beneath it. intake artist merge moves every album from one artist folder into another. A clash -- an album of that name already in the destination -- stops the whole thing before anything moves, rather than leaving one album on top of another.

The audio moves; nothing is copied or deleted. A rename refuses to run when the destination already exists, and tells the user to merge instead.