Makes the 'always recompute' intent explicit by removing the dead-code
short-circuit in add_distance_change — the candidate query already
requires at least one nil pos, so any previously stored distance is
tied to a stale grid/pos pair and must be overwritten.
Safety net for rows that land via direct DB writes (manual fixes, bulk
imports) and bypass Radio.create_contact's grid-resolution requirement.
Runs every hour on the :admin queue with a 500-row per-invocation cap.
Fills whichever side resolves — if one grid is invalid, the other still
gets populated; distance_km is only written when both positions end up
set.
