Add comprehensive security audit report

This security audit identified 12 vulnerabilities across multiple severity levels:
- 3 Critical vulnerabilities (multi-tenant isolation bypass, insecure session management, no CSRF protection)
- 4 High severity issues (missing organization filters, no rate limiting)
- 3 Medium severity issues (inconsistent validation, authorization gaps)
- 2 Low severity issues (debug logs, error message disclosure)

The most critical finding is a multi-tenant data isolation bypass in the opportunities route that allows users to access data from other organizations.

Key recommendations:
1. Immediate: Fix organization validation in opportunities route
2. Immediate: Implement CSRF protection across all forms
3. Short-term: Add rate limiting to prevent abuse
4. Short-term: Implement secure session management with expiration
5. Medium-term: Standardize input validation using Zod

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude