Program forms should support responses editable until, like surveys

Requested by Ropecon 2026 in their post-con feedback.

Surveys have "responses editable until" (`responsesEditableUntil`, exposed in the survey edit page). The program form editor (`[eventSlug]/program-forms/[surveySlug]/edit`) exposes only `activeFrom`/`activeUntil` — program offer editability cannot be given a deadline.

Note the interaction with workflow state: `ProgramOfferWorkflow.response_can_be_edited_by_owner` currently allows owner editing only in state `new` (see #682), so the deadline applies within that window.