weighted_avg_velocity was a naive mean despite the name — a 500-flat ЖК weighed
the same as a 20-flat one. Now count-weighted by flats_total (sql.md AVG
principle): Σ(velocity*flats_total)/Σ(flats_total). Competitors with unknown
flats_total are excluded from weights; if sizes are unknown for ALL, graceful
fallback to the simple mean (den>0 guard). Field name + API contract UNCHANGED
(zero consumer ripple — traced: only CompetitorsSummary, no frontend ref).
Tests: equal sizes → weighted==naive (existing 6.0 stays); NEW test with
500-flat@40 + 20-flat@2 → 38.54 (not naive 21.0), proving the weighting.
