ci: gate jobs by changed paths

.forgejo/workflows/ci.yml

@@ -10,7 +10,30 @@ on:
   pull_request:
 
 jobs:
+  changes:
+    runs-on: docker
+    steps:
+      - uses: actions/checkout@v4
+
+      - id: filter
+        uses: dorny/paths-filter@v3
+        with:
+          filters: |
+            backend:
+              - 'back001/**'
+              - '.forgejo/workflows/**'
+              - 'ci/**'
+            frontend:
+              - 'front001/**'
+              - '.forgejo/workflows/**'
+              - 'ci/**'
+    outputs:
+      backend: ${{ steps.filter.outputs.backend }}
+      frontend: ${{ steps.filter.outputs.frontend }}
+
   backend:
+    needs: changes
+    if: needs.changes.outputs.backend == 'true'
     runs-on: docker
     container:
       image: forgejo.okit.pl/oskar/ci-gradle-node:8.7-jdk17
@@ -31,6 +54,8 @@ jobs:
         run: ./gradlew test build --no-daemon
 
   flutter:
+    needs: changes
+    if: needs.changes.outputs.frontend == 'true'
     runs-on: docker
     container:
       image: forgejo.okit.pl/oskar/ci-flutter-node:stable
@@ -56,4 +81,3 @@ jobs:
       - name: Test
         working-directory: front001/mosenioring
         run: flutter test
-