test: check backend formatting

.gitea/workflows/back.yaml

@@ -0,0 +1,13 @@
+name: check backend
+
+on: [ push, pull_request ]
+
+jobs:
+
+  formatting:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4 # v2 minimum required
+      - uses: axel-op/googlejavaformat-action@v3
+        with:
+          args: "--set-exit-if-changed --aosp"