-- Record whether a block's `observed_at` is a tip observation.
--
-- `RollingWindow::push` has always taken an `at_tip` flag, because the elapsed
-- time between importing two *historical* blocks measures the observer catching
-- up rather than the network producing — divide difficulty by that and the
-- hashrate is wrong by orders of magnitude while still rendering as a number.
-- That flag lived only in memory, so nothing in the database distinguished a
-- block seen as it was mined from one fetched minutes later by a gap fill, and
-- every row in a backfilled batch carries very nearly the same `observed_at`.
--
-- Any query that treats `observed_at` as timing data therefore needs this
-- column. The chain series does not — it derives block interval from
-- `authored_at`, the chain's own clock, which is immune to when we looked — but
-- the propagation and withholding signal documented on `BlockObservation` is
-- precisely `authored_at` minus `observed_at`, and that difference means
-- nothing at all unless the observation was made at the tip.
--
-- Existing rows default to false: most of them *were* tip observations, but
-- which ones is not recoverable, and false reads as "not known to be" — the
-- conservative direction for a flag whose whole job is to stop untrustworthy
-- timings being used as if they were measurements.
alter table block add column at_tip boolean not null default false;