fix(test): add IncrementQuotaUsed to all APIKeyRepository test stubs

Add the missing IncrementQuotaUsed method to:
- fakeAPIKeyRepo (api_key_auth_google_test.go)
- stubApiKeyRepo (api_key_auth_test.go)
- apiKeyRepoStub (api_key_service_delete_test.go)
- authRepoStub (api_key_service_cache_test.go)

backend/internal/service/api_key_service_cache_test.go

@@ -99,6 +99,10 @@ func (s *authRepoStub) ListKeysByGroupID(ctx context.Context, groupID int64) ([]
 	return s.listKeysByGroupID(ctx, groupID)
 }
 
+func (s *authRepoStub) IncrementQuotaUsed(ctx context.Context, id int64, amount float64) (float64, error) {
+	panic("unexpected IncrementQuotaUsed call")
+}
+
 type authCacheStub struct {
 	getAuthCache   func(ctx context.Context, key string) (*APIKeyAuthCacheEntry, error)
 	setAuthKeys    []string