Accept Team Invitation

Seat the logged-in caller into the team a code invites them to.

The counterpart of register()'s team branch for an email that is already an account (backend#323 slice 2). Same code rows, same email-bound lookup — the difference is that the account exists, so the seating goes through :func:app.services.team_service.add_member exactly as the admin console's does: products follow when the move empties the caller's personal team, and the seat limit is enforced under the team-row lock in the same transaction as the seat write (other pending invites never block an accept).