perf(认证): 引入 API Key 认证缓存与轻量删除查询

增加 L1/L2 缓存、负缓存与单飞回源
使用 key+owner 轻量查询替代全量加载并清理旧接口
补充缓存失效与余额更新测试，修复随机抖动 lint

测试: make test

backend/internal/service/admin_service_update_balance_test.go

@@ -0,0 +1,97 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+type balanceUserRepoStub struct {
+	*userRepoStub
+	updateErr error
+	updated   []*User
+}
+
+func (s *balanceUserRepoStub) Update(ctx context.Context, user *User) error {
+	if s.updateErr != nil {
+		return s.updateErr
+	}
+	if user == nil {
+		return nil
+	}
+	clone := *user
+	s.updated = append(s.updated, &clone)
+	if s.userRepoStub != nil {
+		s.userRepoStub.user = &clone
+	}
+	return nil
+}
+
+type balanceRedeemRepoStub struct {
+	*redeemRepoStub
+	created []*RedeemCode
+}
+
+func (s *balanceRedeemRepoStub) Create(ctx context.Context, code *RedeemCode) error {
+	if code == nil {
+		return nil
+	}
+	clone := *code
+	s.created = append(s.created, &clone)
+	return nil
+}
+
+type authCacheInvalidatorStub struct {
+	userIDs  []int64
+	groupIDs []int64
+	keys     []string
+}
+
+func (s *authCacheInvalidatorStub) InvalidateAuthCacheByKey(ctx context.Context, key string) {
+	s.keys = append(s.keys, key)
+}
+
+func (s *authCacheInvalidatorStub) InvalidateAuthCacheByUserID(ctx context.Context, userID int64) {
+	s.userIDs = append(s.userIDs, userID)
+}
+
+func (s *authCacheInvalidatorStub) InvalidateAuthCacheByGroupID(ctx context.Context, groupID int64) {
+	s.groupIDs = append(s.groupIDs, groupID)
+}
+
+func TestAdminService_UpdateUserBalance_InvalidatesAuthCache(t *testing.T) {
+	baseRepo := &userRepoStub{user: &User{ID: 7, Balance: 10}}
+	repo := &balanceUserRepoStub{userRepoStub: baseRepo}
+	redeemRepo := &balanceRedeemRepoStub{redeemRepoStub: &redeemRepoStub{}}
+	invalidator := &authCacheInvalidatorStub{}
+	svc := &adminServiceImpl{
+		userRepo:             repo,
+		redeemCodeRepo:       redeemRepo,
+		authCacheInvalidator: invalidator,
+	}
+
+	_, err := svc.UpdateUserBalance(context.Background(), 7, 5, "add", "")
+	require.NoError(t, err)
+	require.Equal(t, []int64{7}, invalidator.userIDs)
+	require.Len(t, redeemRepo.created, 1)
+}
+
+func TestAdminService_UpdateUserBalance_NoChangeNoInvalidate(t *testing.T) {
+	baseRepo := &userRepoStub{user: &User{ID: 7, Balance: 10}}
+	repo := &balanceUserRepoStub{userRepoStub: baseRepo}
+	redeemRepo := &balanceRedeemRepoStub{redeemRepoStub: &redeemRepoStub{}}
+	invalidator := &authCacheInvalidatorStub{}
+	svc := &adminServiceImpl{
+		userRepo:             repo,
+		redeemCodeRepo:       redeemRepo,
+		authCacheInvalidator: invalidator,
+	}
+
+	_, err := svc.UpdateUserBalance(context.Background(), 7, 10, "set", "")
+	require.NoError(t, err)
+	require.Empty(t, invalidator.userIDs)
+	require.Empty(t, redeemRepo.created)
+}