Add a search table over the whole metadata branch

Add a search table over the whole metadata branch

Search is the other thing the derived rows are for: one document per
issue, change request, and comment, with the title, the body and the
author. FTS5 tokenizes them, so a query ranks by relevance instead of
scanning every body, and `#42` or a ULID prefix is answered from the
numbering table first so the entity someone named comes first.

The documents are maintained by the same refresh as the list rows: an
entity's text is replaced with the entity, a thread's comments are
replaced with the thread, and a deletion removes both. A comment row
carries the parent's title and id, so a result can say what it is on and
link to the fragment without a second read.

The store never decides visibility: Search takes the repository ids the
caller may show, and returns nothing outside them.

This needs SQLite's FTS5 module. The build tag that enables it in the
driver's bundled copy is added to the build commands in the next commit;
a build without it fails at index.Open with that reason.