refactor: flatten monorepo structure to backend/ frontend/ devops/

Rename subdirectories for a cleaner single-repo layout:
- website-monitoring-backend/  → backend/
- website-monitoring-frontend/ → frontend/
- website-monitoring-devops/   → devops/

Update all references in package.json scripts, CI workflows,
docker-compose, pre-commit hooks, and documentation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/vercel.json

@@ -0,0 +1,13 @@
+{
+  "crons": [
+    {
+      "path": "/api/cron/scan?mode=all",
+      "schedule": "0 */6 * * *"
+    }
+  ],
+  "functions": {
+    "app/api/cron/scan/route.ts": {
+      "maxDuration": 300
+    }
+  }
+}