🛡️ fix: Enhance File Upload Security & Error Handling

[danny-avila]

Summary

I implemented comprehensive improvements to file handling and security across upload routes, focusing on avatar uploads, conversation parsing, and error handling.

• Added filename sanitization utility and integrated it into multer storage configuration
• Implemented automatic cleanup of temporary files after processing in image and avatar routes
• Refactored avatar upload handling to use fs.readFile and enhanced file validation checks
• Added proper error handling for conversation parsing in buildEndpointOption middleware
• Enhanced error logging and response handling in checkBan middleware
• Fixed access control for prompt group deletion by validating author permissions
• Refactored avatar upload routes for agents and assistants to use consistent patterns
• Added proper file size validation and mime type checks for avatar uploads
• Fixed cleanup flag handling to ensure proper temporary file deletion
• Improved error messages and logging across file upload routes

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective