Allow disabling registration/RSVP for announcement-only events

When co-hosting events with other communities, registration is often managed on a separate platform (e.g., Eventbrite, Lu.ma, Connpass). In these cases, organizers want to use OCG for event discovery and announcements only — not for registration.

Currently, every event displays an "Attend" button with no way to disable it. This leads to duplicate registration across platforms and confuses attendees about where to actually sign up.

Proposal

Add a registration mode option when creating/editing an event:

• Built-in (default): current behavior
• External: hide the "Attend" button and show a link to an external registration URL
• None: announcement only, no registration at all

The event page itself (title, description, date, location, agenda, etc.) would remain fully functional regardless of the mode.