fix: use scope constant in get_budget permission query (#2510)

* fix: use scope constant in get_budget permission query

The get_budget controller filtered the app_permissions scope column with
models.PAY_INVOICE_METHOD, which only matched because the method and
scope constants share the same string value. Use
constants.PAY_INVOICE_SCOPE like every other scope lookup, and document
why the unchecked First result is safe.

Fixes #2503

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix: return error from get_budget on unexpected permission query failure

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

nip47/controllers/get_budget_controller.go

@@ -2,10 +2,13 @@ package controllers
 
 import (
 	"context"
+	"errors"
 
 	"github.com/getAlby/go-nostr"
 	"github.com/getAlby/hub/db/queries"
+	"gorm.io/gorm"
 
+	"github.com/getAlby/hub/constants"
 	"github.com/getAlby/hub/db"
 	"github.com/getAlby/hub/logger"
 	"github.com/getAlby/hub/nip47/models"
@@ -26,8 +29,21 @@ func (controller *nip47Controller) HandleGetBudgetEvent(ctx context.Context, nip
 	}).Debug("Getting budget")
 
 	appPermission := db.AppPermission{}
-	controller.db.Where("app_id = ? AND scope = ?", app.ID, models.PAY_INVOICE_METHOD).First(&appPermission)
+	result := controller.db.Where("app_id = ? AND scope = ?", app.ID, constants.PAY_INVOICE_SCOPE).First(&appPermission)
+	if result.Error != nil && !errors.Is(result.Error, gorm.ErrRecordNotFound) {
+		logger.Logger.WithFields(logrus.Fields{
+			"request_event_id": requestEventId,
+		}).WithError(result.Error).Error("Failed to fetch pay_invoice permission")
+		publishResponse(&models.Response{
+			ResultType: nip47Request.Method,
+			Error:      mapNip47Error(result.Error),
+		}, nostr.Tags{})
+		return
+	}
 
+	// On ErrRecordNotFound appPermission stays zero-valued and maxAmountSat == 0,
+	// which returns the same empty "no budget" response as a permission with no
+	// budget set.
 	maxAmountSat := appPermission.MaxAmountSat
 	if maxAmountSat == 0 {
 		publishResponse(&models.Response{

nip47/controllers/get_budget_controller_test.go

@@ -207,6 +207,41 @@ func TestHandleGetBudgetEvent_NoBudget(t *testing.T) {
 	assert.Nil(t, publishedResponse.Error)
 }
 
+func TestHandleGetBudgetEvent_DatabaseError(t *testing.T) {
+	ctx := context.TODO()
+	svc, err := tests.CreateTestService(t)
+	require.NoError(t, err)
+	defer svc.Remove()
+
+	nip47Request := &models.Request{}
+	err = json.Unmarshal([]byte(nip47GetBudgetJson), nip47Request)
+	assert.NoError(t, err)
+
+	app, _, err := tests.CreateApp(svc)
+	assert.NoError(t, err)
+
+	dbRequestEvent := &db.RequestEvent{}
+	err = svc.DB.Create(&dbRequestEvent).Error
+	assert.NoError(t, err)
+
+	// simulate a database failure that is not a record-not-found error
+	err = svc.DB.Exec("DROP TABLE app_permissions").Error
+	assert.NoError(t, err)
+
+	var publishedResponse *models.Response
+
+	publishResponse := func(response *models.Response, tags nostr.Tags) {
+		publishedResponse = response
+	}
+
+	NewTestNip47Controller(svc).
+		HandleGetBudgetEvent(ctx, nip47Request, dbRequestEvent.ID, app, publishResponse)
+
+	assert.Nil(t, publishedResponse.Result)
+	require.NotNil(t, publishedResponse.Error)
+	assert.Equal(t, constants.ERROR_INTERNAL, publishedResponse.Error.Code)
+}
+
 func TestHandleGetBudgetEvent_NoPayInvoicePermission(t *testing.T) {
 	ctx := context.TODO()
 	svc, err := tests.CreateTestService(t)