Flexible Invoicing (Decouple Invoice ↔ Booking 1:1)

Goal: As a provider, I want to group multiple bookings on a single invoice so I can bill weekly, monthly, or however works best for me/the client. I may also not want to create an invoice at all for services such as Meet & Greets.

Scope:

• Invoice model supports multiple linked bookings.

• UI for adding a booking to a new invoice, existing invoice, reassigning to a different invoice, or not invoicing at all.

• Maintain backwards compatibility with 1:1 invoices.