Move workshop domain rules out of Admin::WorkshopsController #2938

Problem

Admin::WorkshopsController is the largest controller in the app (~295 lines) and holds domain logic below the presentation layer:

1. Domain validation in the controller (workshop_type_valid? adds a model error from the controller)
2. Deletion policy in the controller (4-hour window + no-invitees rule)
3. Organiser grant/revoke role diffing in private controller methods

Plan

This decomposes into three independent, individually shippable changes, tracked as sub-issues:

• Move the "physical workshop needs a host" validation onto Workshop #2939 — Move the "physical workshop needs a host" validation onto Workshop
• Add Workshop#deletable? and use it in Admin::WorkshopsController#destroy #2940 — Add Workshop#deletable? and use it in Admin::WorkshopsController#destroy
• Extract organiser grant/revoke diffing from Admin::WorkshopsController #2941 — Extract organiser grant/revoke diffing from Admin::WorkshopsController

Each can land separately; no ordering required.