v2.1.1 — TMDB keywords reach an anime library that falls back to TMDB

One correction, reported from the console: useKeywords was refused on any anime
library, so the option was hidden in the library editor and rejected on save.

That rule was right while nothing anime-shaped could produce keywords. It went out of
date in 2.1.0, which let an anime library end its chain with TMDB as a last resort —
and TMDB has keywords.

The rule now follows the sources rather than the library type. useKeywords needs
tmdb among the library's providers, whatever that library is, and the error names the
sources it does read instead of talking about types. In the console the toggle appears
whenever TMDB is in the chain:

{ "library": "Animes", "type": "anime", "providers": ["jikan", "anilist", "tmdb"],
  "useKeywords": true }

It covers exactly the titles TMDB itself answered for. The anime sources above it are
untouched, and they still return their own genres. TMDB can return 50 keywords for one
title, so pair it with overrides.maxGenres — doctor warns when you have not.

Removing TMDB from the chain clears the flag with it, in the editor, so the pair that
cannot work is not one you can build by hand.

Nothing else changes. No configuration that was valid before becomes invalid, and
upgrading is a pull.