🛡️ Security: Add input validation, sanitization, and rate limiting

[ManvithGopu13]

Security Vulnerabilities

Issues:

1. No input validation

   • Prompt length not validated (could cause memory issues)
   • No sanitization of user inputs
   • Model parameter not validated against whitelist

2. No rate limiting

   • API endpoints vulnerable to abuse/DoS
   • No protection against resource exhaustion

3. No file size limits

   • Base64 responses could be extremely large
   • No validation of output file sizes before encoding

4. Path traversal risk

   • Output directory not properly validated
   • Could potentially write outside intended directory

Recommended Fixes:

• Add prompt length validation (max 1000-2000 characters)
• Implement rate limiting using flask-limiter
• Add file size checks before base64 encoding
• Validate and sanitize output directory paths
• Whitelist allowed model names
• Add request timeout handling

Priority: High
Type: Security