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/server/middleware/api_key_auth_google_test.go

@@ -75,6 +75,9 @@ func (f fakeAPIKeyRepo) ListKeysByUserID(ctx context.Context, userID int64) ([]s
 func (f fakeAPIKeyRepo) ListKeysByGroupID(ctx context.Context, groupID int64) ([]string, error) {
 	return nil, errors.New("not implemented")
 }
+func (f fakeAPIKeyRepo) IncrementQuotaUsed(ctx context.Context, id int64, amount float64) (float64, error) {
+	return 0, errors.New("not implemented")
+}
 
 type googleErrorResponse struct {
 	Error struct {