Sso Callback Redirect

Custom SSO callback endpoint that redirects to SuperTokens.

This endpoint:

1. Accepts clean URL path: /auth/sso/callback/{organization_slug}/{provider_slug}
2. Validates the organization and provider exist
3. Builds SuperTokens thirdPartyId: sso:{organization_slug}:{provider_slug}
4. Redirects to SuperTokens callback: /auth/callback/{thirdPartyId}

SuperTokens then handles:

1. Exchange code for tokens (using our dynamic provider config)
2. Get user info
3. Call our sign_in_up override (creates user_identity, adds user_identities to session)
4. Redirect to frontend with session cookie