Alerting: Fix fine-grained rule access control to use 403 for authorization error (#79239)

* use 403 for authorization error
* update silences API
* add ForbiddenError to rule API responses

pkg/services/ngalert/api/tooling/definitions/shared.go

@@ -1,5 +1,7 @@
 package definitions
 
+import "github.com/grafana/grafana/pkg/util/errutil"
+
 // swagger:model
 type NotFound struct{}
 
@@ -11,3 +13,10 @@ type ValidationError struct {
 	// example: error message
 	Msg string `json:"msg"`
 }
+
+// swagger:model
+type ForbiddenError struct {
+	// The response message
+	// in: body
+	Body errutil.PublicError `json:"body"`
+}