-- Next healthcheck due date, recorded against the session that scheduled it.
--
-- Stored as a VARCHAR(10) 'YYYY-MM-DD' calendar date rather than a BIGINT unix
-- timestamp on purpose: this is a diary date a human picked ("look at this site
-- again in October"), not an instant. A timestamp would drag timezone handling
-- into something that has no time-of-day component, and would render as the
-- wrong day for anyone east or west of the server.
--
-- Kept per-session (not on a separate sites table) so the history of what was
-- scheduled when is preserved. A site's *current* next-due is the value on its
-- most recent session — see Store::allSitesSummary().
--
-- Portable: both MySQL 8 and SQLite accept ALTER TABLE ... ADD COLUMN with a
-- nullable typed column and no default. Neither supports ADD COLUMN IF NOT
-- EXISTS in a form the other understands, which is fine — Migrations.php only
-- ever applies each file once.

ALTER TABLE healthchecks ADD COLUMN next_due VARCHAR(10) NULL;