Mark a meeting as booked on a lead

Records that a meeting was booked with this lead. A meeting is an explicit mark set by you or your agent when a call actually gets scheduled - Emailchaser never infers one from reply text or calendars, so this call (or PUT /leads/{id}/category with meeting_booked, which behaves identically) is the only way a meeting is recorded. Sets meetingBookedAt to now, sets the lead's category to meeting_booked, and fires the LeadCategoryUpdate webhook on the category change. Idempotent: repeating the call keeps the original meetingBookedAt and fires no second webhook. Marked meetings are counted in GET /campaigns/{id}/stats totals.meetings and GET /reports/outcomes.