feat(user-service): add family planning email API for batch passenger notifications

Introduce POST /api/user/passenger/planning/family-email with a batch payload (trip context, subject/message, optional trip title & time range, and a list of passengers each with recipient representative IDs). Implement PassengerFamilyPlanningEmailService to render the planning family template and send one email per selected representative per passenger, aggregate sent/failed counts and status, and continue processing when a single passenger fails.