ci: add backend workflow for lint, tests, and audit checks

Author: srija-pixel

.github/workflows/backend.yml

@@ -0,0 +1,31 @@
+name: Backend CI
+
+on:
+  pull_request:
+    paths:
+      - 'backend/**'
+
+jobs:
+  backend-check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Start backend briefly
+        run: |
+          node backend/server.js &
+          sleep 5
+          kill $! || true
+
+      - name: Run security audit
+        run: npm audit || true