JWT SSO via internal user_id only, with no profile updates

Description: As a product team integrating Sleekplan, I want JWT SSO to authenticate users using only our internal user_id (no email required) and to ensure SSO never updates user profile data in Sleekplan, so that users without email can sign in and our stored profiles remain unchanged. Problem: Current JWT SSO flows rely on email as the primary identifier, which forces teams to fabricate dummy emails for users who do not have one. This creates data pollution, adds operational overhead, and risks unintended profile changes when SSO re-syncs data. There is no supported way to use an internal user_id as the sole identifier and guarantee that SSO will not update user fields in Sleekplan. Benefit: Enables authentication for users without email using a stable internal user_id. Eliminates dummy email workarounds and reduces data quality issues. Protects data integrity by decoupling SSO from profile synchronization. Simplifies integration and maintenance while preserving compatibility with existing email-based SSO.