Add createScraperIndexes() to create indexes on the scraper_runs
collection: a compound index on status+startedAt for active job queries
and a descending index on startedAt for recent run lookups.
Change default collection names from production tables
(units_migration_test, unit_prices_migration_test) to dedicated
validation collections (units_scraper, unit_prices_scraper). This
enables the Node.js scraper to run in parallel with the existing Python
scraper during validation without interfering with production data.
Update all scraper test files to reference the new default collection
names.
