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>

CONTRIBUTING.md

@@ -24,8 +24,8 @@ cd website-monitoring
 npm install
 
 # Install project dependencies
-cd website-monitoring-backend && npm install && cd ..
-cd website-monitoring-frontend && npm install && cd ..
+cd backend && npm install && cd ..
+cd frontend && npm install && cd ..
 
 # Start everything
 npm run dev
@@ -71,8 +71,8 @@ npm run dev
 
 ## Testing
 
-- **Backend**: Jest + Supertest — `cd website-monitoring-backend && npm test`
-- **Frontend**: Jest + Testing Library — `cd website-monitoring-frontend && npm test`
+- **Backend**: Jest + Supertest — `cd backend && npm test`
+- **Frontend**: Jest + Testing Library — `cd frontend && npm test`
 - Aim for tests on all API endpoints and critical business logic
 - Use meaningful test names: `it("should return 400 when URL is missing")`