-- Add valid-time columns to entity_facts.
--
-- entity_facts already has created_at — *transaction time*, the
-- moment WE recorded the fact. That's not the same as the real-world
-- period the fact was true. "Cameron is_in_relationship_with X" was
-- only true during a window; recording it in 2026 doesn't make it
-- true today. Without the distinction, every former relationship,
-- former job, former address reads as currently-true.
--
-- Adding two BIGINT NULL columns: valid_from / valid_until (unix
-- seconds). NULL means "unbounded on that side" — `valid_from IS
-- NULL` reads as "always-true-back-to-the-beginning",
-- `valid_until IS NULL` as "still-true-now-or-unknown". Both NULL =
-- temporal validity unknown (current state of all legacy rows).
--
-- Conflict detection refines accordingly: same-predicate facts with
-- different objects stop flagging when their intervals are disjoint
-- ("lives_in NYC 2018-2020" and "lives_in SF 2020-present" are both
-- valid, just at different times).

ALTER TABLE entity_facts ADD COLUMN valid_from BIGINT;
ALTER TABLE entity_facts ADD COLUMN valid_until BIGINT;

-- Optional partial index for time-bounded scans. Skipped for now —
-- conflict detection runs per-entity (small N) and doesn't need it.