fix: address usage email review feedback

Format billing period dates in UTC to match how billing cycle boundaries

are computed, and change IsEntitled to <= limit so users get the full

message allowance and the exceeded email total matches the stated limit.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AchoArnold

api/pkg/emails/hermes_user_email_factory.go

@@ -15,9 +15,10 @@ type hermesUserEmailFactory struct {
 	generator hermes.Hermes
 }
 
-// formatBillingDate renders a date like "19 June 2026"
+// formatBillingDate renders a date like "19 June 2026" in UTC, matching how
+// the billing cycle boundaries are computed in the billing usage repository.
 func formatBillingDate(t time.Time) string {
-	return t.Format("2 January 2006")
+	return t.UTC().Format("2 January 2006")
 }
 
 func (factory *hermesUserEmailFactory) APIKeyRotated(emailAddress string, timestamp time.Time, timezone string) (*Email, error) {

api/pkg/entities/billing_usage.go

@@ -24,7 +24,7 @@ func (usage *BillingUsage) TotalMessages() uint {
 	return usage.SentMessages + usage.ReceivedMessages
 }
 
-// IsEntitled checks if a user can send `count` messages
+// IsEntitled checks if a user can send `count` messages without exceeding `limit`
 func (usage *BillingUsage) IsEntitled(count, limit uint) bool {
-	return (usage.TotalMessages() + count) < limit
+	return (usage.TotalMessages() + count) <= limit
 }