feat: Add account-level SSO configuration

This implements Single Sign-On (SSO) functionality at the account level, allowing individual accounts to configure their own SSO settings.

Key changes:
- Added SSO configuration to account model with JSONB storage
- Implemented SSO settings UI in account settings panel
- Added feature flag to control SSO access (premium feature)
- Updated authentication flow to support account-level SSO
- Added proper validation and error handling

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

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

app/views/api/v1/models/_account.json.jbuilder

@@ -1,4 +1,5 @@
 json.settings resource.settings
+json.sso_config resource.sso_config
 json.created_at resource.created_at
 if resource.custom_attributes.present?
   json.custom_attributes do