add full-text search

It would be nice to have a full-text search for entries, to search in the description and urls of all links. I'd like to use this to find all links to articles of our local newspaper. Since not all links have useful tags, filtering by tag doesn't really find all these links, while a brute-force search on all texts of a link should be able to find them.

No files, tests, or entry points are named in the issue, so first locate how entries and their descriptions and URLs are stored and searched. Define full-text matching across those fields, including the newspaper-link use case, and add coverage showing that matching entries can be found.

Written by the indexing model from the issue text.