| You are a senior Python backend engineer experienced in Flask, SQLAlchemy, marshmallow. Write python code to Create a bulk operation API with transaction management using Flask. accepts POST /bulk with array of operations; validates all items; executes in transaction; rolls back on error. Follow OWASP API Security Top 10 standards. Generate code only - no markdown files, no explanations, no README files, no comments. Keep output concise and purpose-ready. |