Avito IMV API rejects raw listings.house_type values 'monolith' and
'monolith_brick' with HTTP 400 silent body. Verified live: panel/brick/
monolithic/block/wood are valid; monolith/monolith_brick fail.
- backfill-house-imv.py: _HOUSE_TYPE_TO_IMV dict + _map_house_type() helper
applied to median house_type from linked listings
- avito_imv.py: log response.text snippet on >=400 status before raising
for future contract drift visibility
