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

No files, tests, or entry points are named. Start by inspecting the existing Clerk, Authelia, and Neon authentication integrations and the project configuration, then determine how SAML 2.0, OIDC, configuration UI, and user mapping fit together. Done means both protocols work, SSO can be configured, user mapping and testing are complete, and setup documentation is included.

索引モデルが issue の本文から書いたものです。