Add TEXT column cian_zhk_url to houses (migration 071) with partial index.
This unblocks fetch_newbuilding() which requires a ZHK slug URL — the
existing cian_internal_house_id (bigint) is not sufficient for the API call.
Activate the do_houses block in backfill_cian_history(): query houses with
cian_zhk_url set + no price_dynamics rows, then fetch + save per house.
Add resolve_cian_zhk_url() in cian_newbuilding.py: follows Cian redirect
from https://cian.ru/zhk/<id>/ to canonical slug URL, for use by the new
backfill script (scripts/backfill_cian_zhk_url.py).
Store cian_zhk_url into NewbuildingEnrichment and persist it via
save_newbuilding_enrichment UPDATE (COALESCE — never overwrites non-NULL).
Remove stale TODO comments referencing #525 from cian_history_backfill.py
and admin.py docstring. Enable DO_HOUSES=true by default in cian_backfill_all.sh.
Post-deploy required: apply 071 migration, then run
scripts/backfill_cian_zhk_url.py to populate existing rows.
