Act as a senior backend developer with 10+ years of experience in Python and Flask. Create a detailed guide for building a URL shortener using Flask. Include the following steps: [SETTING UP THE FLASK ENVIRONMENT], [DESIGNING THE DATABASE SCHEMA], and [IMPLEMENTING THE URL SHORTENING LOGIC]. Explain how to handle [REDIRECTING SHORTENED URLS] to their original destination. Provide code snippets and best practices for security, such as preventing abuse and validating URLs. Additionally, suggest ways to [OPTIMIZE PERFORMANCE] for high traffic. Ensure the guide is beginner-friendly but includes advanced tips for customization.