Replace bare metro_block placeholder ({"nearest_top3": None}, blocked on a
never-merged 22h metro scraper) with a direct KNN query against osm_poi_ekb
category='metro_stop' (loaded by poi_loader). Returns 3 nearest stations with
name + rounded distance_m; empty list when none nearby (vs None on error),
SAVEPOINT + try/except like adjacent SF-B5 blocks. No frontend consumer yet.
Closes#1667
