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/.taskmaster/state.json

@@ -0,0 +1,6 @@
+{
+  "currentTag": "master",
+  "lastSwitched": "2025-07-24T12:45:40.600Z",
+  "branchTagMapping": {},
+  "migrationNoticeShown": true
+}