Update Referral Slug

PATCH 
/api/business-referral-program/slug

Let the calling sub-account customize their own referral link slug.

The user can only edit the slug on their own affiliate link — the service resolves it from the auth context, so a caller can never touch another tenant's link. Slug is normalized (lowercase, alphanumeric + hyphens) and validated for uniqueness in the default tenant before the write.

Request​