refactor(workshops): move the deletion policy into Workshop#deletable?

[costajohnt]

Closes #2940

Moves the workshop deletion policy out of Admin::WorkshopsController and into the model: Workshop::DELETION_WINDOW (4 hours) and Workshop#deletable?, which is true while the workshop is within that window of created_at and has no invitations. #destroy now calls @workshop.deletable?; the controller constant and its two private helpers are gone. The logic is the same two checks, moved as is, so flash messages and redirects are unchanged.

Testing

• New model specs for #deletable?: just after creation, one minute before the window ends, one minute after it ends, and with an invitation
• Existing spec/controllers/admin/workshops_controller_spec.rb destroy specs pass (the constant reference now points at Workshop::DELETION_WINDOW)
• bundle exec rspec spec/models/workshop_spec.rb spec/controllers/admin/workshops_controller_spec.rb: 106 examples, 0 failures; rubocop clean

[mroderick]

Reviewed and verified in a worktree of the PR branch:

• Workshop#deletable? is the old controller logic moved expression for expression, same check order: invitations.blank? && Time.zone.now.between?(created_at, created_at + DELETION_WINDOW). between? stays inclusive at both ends, so behaviour is unchanged.
• Nothing else references the deleted constant or helpers — the controller specs were the only external consumers and are updated to Workshop::DELETION_WINDOW.
• New model specs cover both sides of each condition: just after creation, one minute inside the window, one minute outside, and with an invitation.
• Ran bundle exec rspec spec/models/workshop_spec.rb spec/controllers/admin/workshops_controller_spec.rb: 106 examples, 0 failures. rubocop on both app files: clean.

Non-blocking nit: DELETION_WINDOW sits between resourcify and the has_many block rather than with the other constants at the top of the model — cosmetic only.