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>

README.md

@@ -77,14 +77,14 @@ cd website-monitoring
 npm install
 
 # Setup backend
-cd website-monitoring-backend
+cd backend
 cp .env.example .env
 npm install
 npm run build
 cd ..
 
 # Setup frontend
-cd website-monitoring-frontend
+cd frontend
 cp .env.example .env    # Fill in your Supabase keys
 npm install
 cd ..
@@ -109,7 +109,7 @@ npm run docker:up
 
 ```
 website-monitoring/
-├── website-monitoring-backend/    # Express.js API + Lighthouse engine
+├── backend/    # Express.js API + Lighthouse engine
 │   ├── src/
 │   │   ├── index.ts              # Server entry, health check, routing
 │   │   └── routes/
@@ -117,7 +117,7 @@ website-monitoring/
 │   ├── Dockerfile
 │   └── package.json
 │
-├── website-monitoring-frontend/   # Next.js 15 dashboard
+├── frontend/   # Next.js 15 dashboard
 │   ├── src/
 │   │   ├── app/                  # Pages & API routes (20+ endpoints)
 │   │   ├── components/           # React components (dashboard, UI, auth)
@@ -126,7 +126,7 @@ website-monitoring/
 │   ├── Dockerfile
 │   └── package.json
 │
-├── website-monitoring-devops/     # Infrastructure
+├── devops/     # Infrastructure
 │   ├── docker-compose.yml        # Full stack orchestration
 │   └── .devcontainer/            # VS Code Dev Container config
 │