Replace unweighted AVG(deals_total_avg_area_m2) and AVG(deals_total_avg_price_thousand_rub_per_m2)
with SUM(x * count) / NULLIF(SUM(count), 0) pattern in _INLINE_VELOCITY_SQL.
Months with zero deals no longer dilute the weighted mean 2-4x.
P0 follow-up to PR #290 (mv fix, issue #21).