Accept Invitation

Accept a team invitation.

Validates the invitation belongs to the user's email.

Payment is intentionally NOT enforced (enforce_payment=False): accepting an invite consumes no credits and is the canonical path for a user on the unpaid free plan to join a paid team. Gating it behind the billing check blocked free-plan invitees from ever joining (min_plan must be free for this route — see docs/endpoint-access-matrix.md).