feat(stacks): use source id to create git stacks [BE-13043] (#2870)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

api/stacks/stackbuilders/stack_payload.go

@@ -36,8 +36,11 @@ type StackPayload struct {
 }
 
 type RepositoryConfigPayload struct {
+	// SourceID references an existing Source.
+	// When non-zero, only ReferenceName is still applied.
+	SourceID portainer.SourceID
 	// URL of a Git repository hosting the Stack file
-	URL string `example:"https://github.com/openfaas/faas" validate:"required"`
+	URL string `example:"https://github.com/openfaas/faas"`
 	// Reference name of a Git repository hosting the Stack file
 	ReferenceName string `example:"refs/heads/master"`
 	// Use basic authentication to clone the Git repository