Naive backend for tests, for a topic-only listing and for any storage without a native index.
It matches a note when any word is a substring of its title, topics or body, and ranks by word
hits weighted title 3, topics 2, body 1. It scans the scope, bounded to the 500 most recently
updated notes, so it is a fallback and not a way to run a large store. It matches substrings
(log matches catalog), where the other backends match whole words.