Set how many days a firm's interval contexts are kept (operators only)

Sets policy.context_retention_days (1 to 365; zero retention is not built) and, in the same transaction, moves every stored expiry of the firm's device contexts and notes, decided cells' screen material, runs, steps, derivation cache and stored calendar by the difference in days, never recomputing it, so a shortening applies at once. Decisions and entries have no expiry and are not touched. Answers the counts moved, the old tables' included: they move too until the follow-up drops them.