Follow-up hardening from PR #858 review. The quarter-index factor
(target_index / avg_analog_index) was unbounded — pathological FDW data could
yield an extreme multiplier on a client-visible price. Clamp factor to
[estimate_quarter_index_factor_min, _max] = [0.6, 1.8] (outside the realistic
quarter-to-quarter location spread for EKB вторичка). Bounds in settings;
passed as kwargs so _apply_quarter_index stays pure/testable. Clamp fires
logged at INFO. Formula and #764 guards unchanged.
Refs #859, #764
