💳 Set Who Pays on Family and Couples Contracts

We're adding a designated billing contact to family and couples contracts, so the person responsible for payment is set once on the contract instead of chosen invoice by invoice.

Today you pick which client an invoice goes to each time you bill. That works, but it's a decision you repeat for a fact that never changes, and it leaves room for the bill reaching the wrong person. With a billing contact on the contract, invoices, superbills, and payment reminders go to the right family member automatically, and everyone else stays out of billing they aren't responsible for.

We're also working on insurance for multi-participant contracts, so claims can be filed under the family member who carries the policy rather than only from an individual contract.