InvitationManager silently no-ops on non-invitable events/workshops while controllers flash success #2945

Problem

Three InvitationManager methods guard with error strings — send_event_emails, send_workshop_emails, send_virtual_workshop_emails all return 'The X is not invitable' unless x.invitable?. All three run via handle_asynchronously, so the return value is discarded by the DelayedJob worker. The controllers (Admin::WorkshopsController#invite, Admin::EventsController#invite) ignore the return anyway and flash "Invitations will be emailed out soon." unconditionally.

Net effect: when the target isn't invitable, nothing is sent and nobody learns — the flash lies.

Proposed change

• Check invitable? in the controllers before enqueueing and flash a warning instead of the success message when it's false. (Raising inside the async method can't fix the flash — the raise happens after the redirect.)
• Remove the now-dead string returns from InvitationManager; the internal batch-abort strings (start_invitation_batch → return result if result.is_a?(String)) may stay as internal control flow — implementer's call.

Acceptance

• Inviting to a non-invitable event/workshop shows a warning flash and enqueues nothing.
• Inviting to an invitable one behaves as today.