Migration 133 FAILED on prod: offer_price_history has UNIQUE (listing_id,
change_time), but the re-point UPDATE moved ALL loser rows to the keeper →
duplicate key violation (the deep-review validated on a throwaway schema subset
that lacked this constraint). A keeper-only NOT EXISTS guard is also insufficient
— multiple losers in one (source,source_id) group share a change_time, so they
collide WITH EACH OTHER inside the single UPDATE (same vulnerability for
listings_snapshots PK + external_valuations partial-UNIQUE).
Fix: drop all data-child re-points; rely on FK ON DELETE CASCADE (all four FKs
are confdeltype='c') to clean loser children when losers are deleted. Keep only
the collision-free merged_into re-point. These are duplicates of the SAME offer —
the keeper retains its own full history; loser-dup history is redundant.
Dry-run on prod (BEGIN…ROLLBACK): 9840 groups, 9889 losers deleted, index
created, abort-guard passed, zero errors. Migration 133 never applied anywhere
(absent from _schema_migrations), so amending it in place is safe.
Refs #1773
