Fix invitation re-sends and seat limit gaps

Re-inviting a pending user, or inviting through the authentication API rather than the dashboard, did not follow the same rules as a regular invitation.

• Fixed re-inviting an already-invited user: the link already shared stays valid and its validity is extended; changing the role or access group now issues a fresh invitation instead
• Fixed plan seat limits and the role ceiling not applying to invitations created through the authentication API
• Unified invitation validity at 30 days across every invite path