Invite people to a project

Invite one or more people (body: { "emails": [...] }) to the project given in the required project header. Each invitation is created with the user role, or AGENT when the inviter's role could not grant user, and an invitation email is sent. Returns the subset of submitted email addresses that were SKIPPED because they already belong to the project or its organisation; an empty array means every address was invited. Fails with 403 if the inviter's role cannot grant even AGENT, and with a plan error if the organisation cannot seat that many additional teammates.