Program forms should support responses editable until, like surveys

[japsu]

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.