Tiered pricing for Group Bookings: base price + per-additional-attendee rules (incl. tiers + per-duration)

Problem / Why it matters: Group Bookings commonly needs pricing that is not simply “price × attendees.” Many businesses price a private session as: * a base rate covering the first attendee (or first “seat”), and * a reduced marginal rate for each additional attendee, sometimes with tier breaks (e.g., the 5th+ person is cheaper), and often dependent on duration. Right now, operators end up using awkward workarounds (service extras, separate services for each headcount, manual invoicing) which increases admin effort and creates customer confusion. Real-world use case (example): Private photography lesson (capacity 1–8): * Minimum booking duration: 2 hours (in-person) * Price model (per hour): * 1 attendee: $150/hr * each additional attendee: +$50/hr * optionally: after 4 attendees, additional attendees become +$40/hr (tiered discount) Desired solution: Add a Group Pricing Rules configuration for a service that can calculate price based on: 1. Attendees (seat count) 2. Duration (minutes / hours) 3. Tier rules (ranges of attendees with different marginal rates) A flexible model could be: * Base price (applies once, e.g., includes first attendee) * Additional attendee pricing using rule tiers, each tier supporting either: * Fixed amount (e.g., +$50 per extra attendee per hour), or * Percentage-based (e.g., +30% of base per extra attendee), or * Custom “per attendee” fixed amount per booking (not time-based)