* Fix critical security vulnerabilities
Address 5 critical findings from security audit:
- Sanitize custom embed HTML to prevent stored XSS (strip all non-iframe content)
- Escape URLs in format_body/1 to prevent reflected XSS via post messages
- Add authorization check to form export endpoint (IDOR fix)
- Replace String.to_atom/1 on user input with explicit whitelists (8 locations)
- Add IP-based rate limiting on authentication endpoints via Hammer
* Start rate limiter before endpoint in supervision tree
* Update CHANGELOG
