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>

backend/.env.example

@@ -0,0 +1,11 @@
+# Server
+PORT=5000
+
+# Database (PostgreSQL)
+DATABASE_URL=postgresql://user:password@localhost:5432/monitoring
+
+# CORS
+CORS_ORIGIN=http://localhost:3000
+
+# Chrome (for Docker/CI)
+CHROME_PATH=/usr/bin/chromium