Admin: set referral slug

Set a wallet's referral vanity slug on their behalf. Only allowed when address already has a row in affiliates — this is an affiliate-onboarding override, not a general account tool. Same grammar as the self-service UpdateReferralSlug (3-32 chars, lowercase/digits/hyphen/underscore), but skips the EIP-191 signature since admin auth substitutes for wallet-ownership proof. Pass an empty slug to generate a random one.