Update credit grant priority so that referrals come before ads

Author: jahooma

common/src/constants/grant-priorities.ts

@@ -2,8 +2,8 @@ import type { GrantType } from '@codebuff/common/types/grant'
 
 export const GRANT_PRIORITIES: Record<GrantType, number> = {
   free: 20,
-  ad: 30, // Ad credits consumed after free, before referral
-  referral: 40,
+  referral: 30,
+  ad: 40,
   admin: 60,
   organization: 70,
   purchase: 80,

packages/billing/src/billing.knowledge.md

@@ -48,7 +48,8 @@ Only last grant can go negative. No maximum debt limit enforced in code.
 ## Grant Types and Priorities
 
 - free (20): Monthly free credits
-- referral (40): Referral bonus credits
+- referral (30): Referral bonus credits (one-time bonuses, consumed before renewable ad credits)
+- ad (40): Ad impression credits (renewable source, consumed after referral)
 - admin (60): Admin-granted credits
 - organization (70): Organization credits
 - purchase (80): Purchased credits