Create a partner (Greentally staff only)

Requires the platform capability partner.ops (403 PLATFORM_CAPABILITY_REQUIRED; API keys and impersonation sessions are always refused); 404 while PARTNERS_ENABLED is off. With clerkOrganizationId, registers that Clerk organization (409 PARTNER_HOLDS_DATA when its tenant has business organizations, ALREADY_PARTNER, ALREADY_CLIENT); without it, creates one with the instance default member limit memberships and the caller as admin. Members are added to Core at once and to Clerk after; memberErrors lists Clerk refusals. Audited (partner.create).