Two scrapers calling match_or_create_house() concurrently for the same address
could both miss Tier 0-3 and each INSERT a new canonical house — splitting
listings across duplicate house rows and breaking aggregations.
Serialize via pg_advisory_xact_lock(42, hashtext(fingerprint)). Lock is
transaction-scoped (released on caller's COMMIT/ROLLBACK). After unblock, the
losing scraper's Tier 2 fingerprint lookup hits the winner's alias and returns
the same house_id.
Closes finding #1 from 2026-05-24 trade-in audit.
