fix ci for external PRs

.github/workflows/backend.yml

@@ -1,5 +1,8 @@
 name: backend
 on:
+  pull_request:
+    branches:
+      - "master"
   push:
     branches:
       - "**"

.github/workflows/frontend.yaml

@@ -1,5 +1,8 @@
 name: frontend
 on:
+  pull_request:
+    branches:
+      - "master"
   push:
     branches:
       - "**"