fix: harden usage billing idempotency and backpressure

backend/internal/repository/usage_log_repo_unit_test.go

@@ -3,8 +3,11 @@
 package repository
 
 import (
+	"strings"
 	"testing"
+	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/service"
 	"github.com/stretchr/testify/require"
 )
 
@@ -39,3 +42,26 @@ func TestSafeDateFormat(t *testing.T) {
 		})
 	}
 }
+
+func TestBuildUsageLogBatchInsertQuery_UsesConflictDoNothing(t *testing.T) {
+	log := &service.UsageLog{
+		UserID:       1,
+		APIKeyID:     2,
+		AccountID:    3,
+		RequestID:    "req-batch-no-update",
+		Model:        "gpt-5",
+		InputTokens:  10,
+		OutputTokens: 5,
+		TotalCost:    1.2,
+		ActualCost:   1.2,
+		CreatedAt:    time.Now().UTC(),
+	}
+	prepared := prepareUsageLogInsert(log)
+
+	query, _ := buildUsageLogBatchInsertQuery([]string{usageLogBatchKey(log.RequestID, log.APIKeyID)}, map[string]usageLogInsertPrepared{
+		usageLogBatchKey(log.RequestID, log.APIKeyID): prepared,
+	})
+
+	require.Contains(t, query, "ON CONFLICT (request_id, api_key_id) DO NOTHING")
+	require.NotContains(t, strings.ToUpper(query), "DO UPDATE")
+}