Delete user referrer relationship

Removes the user_referrers row for a user in the project derived from the API key.

Authentication:

• Requires API key with service_role scope in the Authorization header: Authorization: Bearer <api_key>

Use cases:

• Clear admin-imported referrer assignments created via PUT without a referral code redemption
• Undo update_user_referrer / PUT assignments when remove_user_from_referral_code cannot apply (no matching referral code use)

Behavior:

• Orphaned referral_code_uses rows (referrer/code no longer matches current user_referrers) are deleted automatically and actual_uses is decremented
• If the user has matching referral_code_uses and force is not set, returns 422 — use DELETE /v1/referral_codes/{code}/referrals instead
• With force=true, deletes all referral_code_uses for the user (matching or orphaned), decrements actual_uses, then deletes user_referrers
• If no row exists, returns 422 User referrer relationship not found