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/tests/api/alerting/api_ruler_test.go

@@ -285,7 +285,7 @@ func TestIntegrationAlertRulePermissions(t *testing.T) {
 				ExportQueryParams: apimodels.ExportQueryParams{Format: "json"},
 				FolderUID:         []string{"folder2"},
 			})
-			assert.Equal(t, http.StatusUnauthorized, status)
+			assert.Equal(t, http.StatusForbidden, status)
 		})
 
 		t.Run("Export from one group", func(t *testing.T) {