Create User Event

Create or update a user's association with an event.

• Catalog attach (event_id provided): looks up the existing Event and attaches via UserEvent.attach.
• Prefilled attach (no event_id): materializes an Event(data_quality=prefilled) from the prefilled payload, attaches, and dispatches a DiscoverySession(prefilled_confirm) so the discovery worker enriches it asynchronously.

replace_existing (default True) preserves the legacy single-goal flow that old mobile builds depend on — every other planned/active row is cancelled before the new one is inserted. Calendar-aware clients send False to append.