Enrich competitor_rows query with pricing from objective_lots via
objective_complex_mapping JOIN backbone (domrf_obj_id → project_name).
New fields in competitors[]: avg_price_per_m2_rub (81% coverage vs 0.3%
from domrf_kn_flats), avg_area_pd, units_sold, units_available,
lots_with_price. LEFT JOIN preserves competitors without mapping (fields
= NULL), no regression on existing competitor count/order.
Also fix test_velocity.py: update _sales_row mock with has_mapping field
and VelocityResult constructor with velocity_data_available/velocity_source
fields introduced in OBJ-2 (velocity.py had diverged from test mocks).
All 33 velocity + gate_verdict tests pass.
Closes#307 (OBJ-3 sub-task)
