Migration 079 adds listing_source_snapshots (daily per-source snapshot, plain
table — partitioning deferred), listing_source_events (change-log), an
offer_price_history.listing_source_id backlink, and v_listing_source_price_on_date.
snapshot_listing_sources() (app/tasks) writes a daily snapshot of the existing
18,355 listing_sources rows and diffs today's price vs the prior snapshot into
price_change events — both set-based SQL (CTE), no row loop. Wired into the
in-app scheduler (source='listing_source_snapshot', enabled=true: pure internal
DB, no external calls, window 01:00-02:00 UTC before the backfill jobs). History
accumulates immediately, independent of the dormant scrapers.
Prod dry-run (rollback): migration applies clean; snapshot inserts 18,355 rows.
Same-day double-fire impossible (_claim_run advances next_run_at + has_running_run
guard) so no duplicate price_change events.
Closes#570
