ekb_districts.median_price_per_m2 was populated from a 24-month window.
LEFT JOIN mv_quarter_price_per_m2 on the cad quarter and COALESCE(median_12m,
fallback) so the API returns the 12-month median where available.
Frontend OverviewTab adds a "(12 мес)" hint next to the value.
Closes#271 item 7