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>

.github/workflows/backend.yml

@@ -4,16 +4,16 @@ on:
   push:
     branches: [main, develop]
     paths:
-      - "website-monitoring-backend/**"
+      - "backend/**"
       - ".github/workflows/backend.yml"
   pull_request:
     branches: [main]
     paths:
-      - "website-monitoring-backend/**"
+      - "backend/**"
 
 defaults:
   run:
-    working-directory: website-monitoring-backend
+    working-directory: backend
 
 jobs:
   lint-test-build:
@@ -31,7 +31,7 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
           cache: npm
-          cache-dependency-path: website-monitoring-backend/package-lock.json
+          cache-dependency-path: backend/package-lock.json
 
       - name: Install dependencies
         run: npm ci
@@ -52,5 +52,5 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: backend-coverage
-          path: website-monitoring-backend/coverage/
+          path: backend/coverage/
           retention-days: 7