fix(quota): 채팅 쿼터 구매

- nextRechargeAt = null 설정

src/main/kotlin/kr/co/vividnext/sodalive/chat/quota/ChatQuotaService.kt

@@ -76,5 +76,6 @@ class ChatQuotaService(
     fun purchase(memberId: Long, addPaid: Int) {
         val quota = repo.findForUpdate(memberId) ?: repo.save(ChatQuota(memberId = memberId))
         quota.remainingPaid += addPaid
+        quota.nextRechargeAt = null
     }
 }