Add rustfmt CI check

.github/workflows/backend-checks.yml

@@ -15,6 +15,17 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  rustfmt:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --manifest-path ./src-tauri/Cargo.toml --all -- --check
+
   clippy:
     runs-on: ${{ matrix.platform }}