Create group invite

Invite an email or wallet address into the group's tenant and, on acceptance, into this group. Same rules as POST /tenants/{id}/invites with group_ids=[id]: provide exactly one of email or wallet_address; callers without tenant authority always invite at the "user" role (another role returns 403). Gated like POST /groups/{id}/members: 403 if the caller may not add members to the group, 404 for a missing, retired or other-tenant group. 403 verified_email_required when the caller signed up with only a passkey and has not verified an email yet.