Update a person's contract with PATCH /people/{personId}/contracts/{contractId}

You can now update an existing contract in place with PATCH /people/{personId}/contracts/{contractId}, instead of deleting and recreating it. All contract fields are optional, so you can send just the ones you want to change — startDate, endDate, minutesPerDay, rosteredDays, employmentType, costPerHour, jobTitle and roleId. Passing endDate: null clears an end date, and setting one on an ongoing contract is what lets the person be archived via PATCH /people/{personId}. Invalid updates return a 400 describing the problem (for example a minutesPerDay that doesn't match the rostered days), and a contract that doesn't exist or belongs to another person returns a 404.