Add a note to a booking

Adds a private internal note to a booking. Notes are visible to staff members only and are never shown to customers. Use them to log internal observations, follow-up activity, or any staff-only context tied to the booking.

Each call appends a new note record with its own author and timestamp (a running timeline), which is distinct from the booking's free-form public_notes / private_notes text fields on the event itself (those are edited via PUT /api/v1/events/{event_id}).

Optionally set or update the booking's next follow-up date in the same call.

Required permission: assigned_event_read or unassigned_event_read