fix(认证): 补齐余额与删除场景缓存失效

为 Usage/Promo/Redeem 注入认证缓存失效逻辑
删除用户与分组前先失效认证缓存降低窗口
补充回归测试验证失效调用

测试: make test

backend/internal/service/auth_cache_invalidation_test.go

@@ -0,0 +1,31 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestUsageService_InvalidateUsageCaches(t *testing.T) {
+	invalidator := &authCacheInvalidatorStub{}
+	svc := &UsageService{authCacheInvalidator: invalidator}
+
+	svc.invalidateUsageCaches(context.Background(), 7, false)
+	require.Empty(t, invalidator.userIDs)
+
+	svc.invalidateUsageCaches(context.Background(), 7, true)
+	require.Equal(t, []int64{7}, invalidator.userIDs)
+}
+
+func TestRedeemService_InvalidateRedeemCaches_AuthCache(t *testing.T) {
+	invalidator := &authCacheInvalidatorStub{}
+	svc := &RedeemService{authCacheInvalidator: invalidator}
+
+	svc.invalidateRedeemCaches(context.Background(), 11, &RedeemCode{Type: RedeemTypeBalance})
+	svc.invalidateRedeemCaches(context.Background(), 11, &RedeemCode{Type: RedeemTypeConcurrency})
+
+	require.Equal(t, []int64{11, 11}, invalidator.userIDs)
+}