Accept agent invitation
Links the invited athlete to the inviting agent. A pending athlete may accept (ENG-1357): representation is allowed during onboarding, and approval is only required to be applied to a deal. An unapproved athlete may not. Representation is exclusive, so accepting one invitation deletes every competing invitation for the same email. Invitations expire after 7 days; legacy rows with no expiry stay acceptable rather than being retroactively invalidated. The invitation must be addressed to the signed-in athlete’s email (case-insensitive); anyone else’s token is refused as INVITATION_INVALID. An athlete another agent already represents is refused with ATHLETE_REPRESENTED_BY_OTHER_AGENT (409). Once the link is committed the agent gets notification 51 and the athlete the Athlete now represented by Agent confirmation (ENG-1568 / ENG-1569); accepting an invitation for an agent who already represents the athlete sends nothing.