Creates or overwrites a referrer-referee relationship via API.

Authentication:

• Requires API key with service_role scope in the Authorization header: Authorization: Bearer <api_key>
• The project is automatically derived from the API key's associated project

Use cases:

• Clients that manage their own referral system (fintechs, CEXs) — sync existing referral relationships with Fuul without using SDK or referral codes flow
• Projects that need manual control over referral relationships — override, correct, or manually set who referred whom

Behavior:

• If (project, user_identifier) does NOT exist in user_referrers → create the relationship
• If it already exists → overwrite the referrer (replace existing)
• If referral_code is provided, the referral code will be looked up and its rebate_rate will be locked into the user_referrers relationship

Supports all identifier types (email, evm_address, solana_address, etc.).