domrf_kn_flats.status is NULL in ~99.8% of rows, so WHERE status='sold'
always returned 0 rows and avg_price_per_m2 was always None. Drop the
filter; AVG over all rows with price_per_m2 IS NOT NULL is semantically
correct for a complex-level price estimate.
Adds regression test test_competitors_avg_price_populated (Issue #227).
