Add markStaleUnits() that marks units as unavailable when they are not
present in the current scrape results. Uses MongoDB updateMany with a
$nin filter on unit_code to efficiently update only units that were
previously available but no longer appear in the scraped data. Sets
available to false and records marked_stale_date.
Includes 18 tests covering filter criteria, $set fields, already
unavailable units, empty currentUnitCodes, error handling, logging,
and return value verification.
