Move the "physical workshop needs a host" validation onto Workshop #2939

Parent: #2938

Problem

Admin::WorkshopsController enforces the rule "a physical workshop must have a host" in the controller (workshop_type_valid? plus physical_workshop_and_no_host?), adding a validation error from the controller:

def workshop_type_valid?
  return true unless physical_workshop_and_no_host?(@workshop)

  @workshop.errors.add(:host, "can't be blank")
  false
end

Validation rules belong on the model so create, update, and any future caller get them for free.

Proposed change

• Move the rule into a Workshop validation, reusing the controller's intent and error message (host "can't be blank").
• Delete workshop_type_valid? and physical_workshop_and_no_host? from the controller; the create/update flows keep their existing flash behaviour via @workshop.errors.

Acceptance

• Creating/updating a physical workshop without a host fails with the same error message as today.
• Model specs cover the new validation; existing admin workshop controller specs pass.