Security, code quality and frontend improvements

- Move SECRET_KEY out of docker-compose into .env (env_file), add .env.example
- Add flask-limiter with 10 req/min on login route; introduce util/limiter.py
- Replace direct mysql.connector.connect() calls with MySQLConnectionPool via util/db_pool.py
- Fix deprecated datetime.utcnow() -> datetime.now(timezone.utc) in auth/login.py
- Remove dead /api/scripts 410 route from admin.py
- Add MD5 security warning in Md5Tool.jsx
- Add ErrorBoundary component and wrap App.jsx
- Expand README with setup guide, screenshot and project structure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.dev.yml

@@ -5,9 +5,8 @@ services:
       dockerfile: Dockerfile
     ports:
       - "5050:5000"
-    environment:
-      - SECRET_KEY=dev-change-me
-      - DB_CONFIG_PATH=/config/db_config.json
+    env_file:
+      - .env
     volumes:
       - config-data:/config
       - logs-data:/app/backend/logs