Create Team Invitation

Invite someone into the caller's team by email.

Two deliberate differences from the personal POST /api/auth/invitations:

• No email_is_registered 409. A personal invite is only a way to register, so inviting an existing user is a dead code. A team invite is a seat: an existing account accepts the very same code while logged in (POST /api/auth/invitations/accept) and moves in, products following.
• No PER_USER_TOTAL_QUOTA. That cap throttles viral registration growth; a team invite is bounded by something better — the seats the team's plan actually has (the ensure_seat_available call below).

Member-only on purpose: invited_team_role is pinned to team_member here, promotions stay a staff action in the admin console.