Security: Fix SSRF vulnerability in attach_photo_from_url

[bellisabell]

Summary

Fixes a critical SSRF vulnerability in attach_photo_from_url and attach_cover_from_url methods where URI.open(url) was used without any validation.

Changes

• URL scheme validation: Only allow http:// and https:// URLs (blocks file://, ftp://, etc.)
• Timeout constraints: 10-second open/read timeouts using Net::HTTP instead of URI.open
• Content-type validation: Only allow image types (image/jpeg, image/png, image/gif, image/webp)
• File size validation: Maximum 10MB attachment size
• Safe redirect handling: Follow up to 3 redirects, validating scheme on each

Security Impact

Previously an attacker could:

• Access internal services via http://localhost:... or http://169.254.169.254/... (cloud metadata)
• Read local files via file:///etc/passwd
• Cause DoS via slow/infinite responses or huge files

Now all these vectors are blocked.

Closes #16