When no comps/anchor are found the estimate returns median=0/range 0-0, and
the frontend can render a literal «0 ₽». Add a computed boolean
insufficient_data (= median_price_rub <= 0) so the UI shows an explicit
«недостаточно данных» state instead.
Implemented as a Pydantic @computed_field derived from median_price_rub —
correct automatically across all four AggregatedEstimate construction sites
(main estimate, _empty_estimate, and both GET-rehydration paths), no DB column,
no risk of per-site drift. Additive/serialized; existing fields untouched.
Refs #697
