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/docker-compose.yml

@@ -0,0 +1,18 @@
+version: "3.8"
+services:
+  frontend:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    ports:
+      - "3000:3000"
+    env_file:
+      - .env
+
+  scan-worker:
+    build:
+      context: ./scanner-worker
+      dockerfile: Dockerfile
+    ports:
+      - "5001:5001"
+    restart: unless-stopped