mailcore: full-text search indexes HTML markup of HTML bodies #1905

Found while planning #1901 (mailcore: the engine can now fetch HTML bodies with
EngineOptions { body_format: BodyFormat::Html }).

The bodies_fts_ai / bodies_fts_au triggers (mailcore/src/store/schema.rs) copy
bodies.content verbatim into messages_fts.body. With an Html engine most stored bodies
are HTML, so Store::search matches tag names, attribute values, URLs and <style> CSS
(font, color, div, table hit almost every message). Local html drafts already have
this today; lookxy (Text) is unaffected.

Fix idea: index a text rendering of html bodies (htmlrender already turns HTML into text).
SQLite triggers cannot call Rust unless a function is registered on the connection, so either
register a body_text() SQL function used by the triggers, or maintain messages_fts from
put_body / put_fetched_body / the draft writers. The triggers are CREATE TRIGGER IF NOT EXISTS, so existing stores need a DROP/recreate migration plus a reindex of html bodies.

Source: #1901, PR #1904
Severity: minor; origin: plan v1