feat(billing): convert referral bonuses from monthly recurring to one-time

- Add is_recurring column to referral table with migration and backfill
- Update calculateTotalReferralBonus to filter by is_recurring=true AND status=completed
- Update redeemReferralCode to grant one-time credits with expiresAt: null
- Add CreditExpirationBucket type and breakdownByExpiration/principalsByExpiration to CreditBalance
- Extract createInitialCreditBalance helper to eliminate DRY violation
- Update UsageDisplay to use data-driven expiration buckets
- Update UI messaging to remove per month references
- Add expiration-buckets tests including edge cases for zero/negative balances

Author: brandonkachen

cli/src/commands/referral.ts

@@ -50,8 +50,8 @@ export async function handleReferralCode(referralCode: string): Promise<{
     const postUserMessage: PostUserMessageFn = (prev) => [
       ...prev,
       getSystemMessage(
-        `🎉 Noice, you've earned an extra ${creditsRedeemed} credits!\n\n` +
-          `(pssst: you can also refer new users and earn ${CREDITS_REFERRAL_BONUS} credits for each referral at: ${env.NEXT_PUBLIC_CODEBUFF_APP_URL}/referrals)`,
+        `🎉 Noice, you've earned ${creditsRedeemed} bonus credits!\n\n` +
+          `(pssst: you can also refer new users and earn ${CREDITS_REFERRAL_BONUS} credits per referral at: ${env.NEXT_PUBLIC_CODEBUFF_APP_URL}/referrals)`,
       ),
     ]
     return { postUserMessage }