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/supabase/migrations/20250813102541_fix_crawl_sessions_progress.sql

@@ -0,0 +1,6 @@
+-- Fix crawl_sessions table by adding missing progress_percentage column
+-- Add missing progress_percentage column to crawl_sessions table
+ALTER TABLE crawl_sessions ADD COLUMN IF NOT EXISTS progress_percentage INTEGER DEFAULT 0;
+
+-- Refresh schema cache to pick up new column
+NOTIFY pgrst, 'reload schema';