Implement Single Sign-On (SSO) Integration

## Overview
Add SSO support using SAML 2.0 and OIDC to enable enterprise authentication integration.

## Business Value
SSO is required for enterprise customers who need to integrate with their identity providers.

## Current State
- Multiple auth providers exist (Clerk, Authelia, Neon)
- No SSO/SAML support
- No OIDC support

## Subtasks
- [ ] Implement SAML 2.0 support
- [ ] Implement OIDC support
- [ ] Add SSO configuration UI
- [ ] Implement SSO user mapping
- [ ] Add SSO testing
- [ ] Document SSO setup

## Implementation Steps
1. Add SAML library
2. Implement SAML handler
3. Add OIDC support
4. Create SSO configuration
5. Test with providers

## Acceptance Criteria
- [ ] SAML 2.0 works
- [ ] OIDC works
- [ ] SSO can be configured
- [ ] User mapping works
- [ ] Documentation is complete

## Estimated Effort
- Hours: 32-40
- Complexity: High