chore: add GitHub Actions keep-alive ping for Render backend

Pings backend every 10 minutes so Render free tier never sleeps.
Reads BACKEND_URL from GitHub Actions secret.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/keep-alive.yml

@@ -0,0 +1,14 @@
+name: Keep Backend Alive
+
+on:
+  schedule:
+    - cron: '*/10 * * * *'  # every 10 minutes
+  workflow_dispatch:
+
+jobs:
+  ping:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Ping backend
+        run: |
+          curl -sf ${{ secrets.BACKEND_URL }} || echo "Backend ping failed"