[Improvement] Migration Proof Notification Template Manager

[Thumimku]

Describe the issue: When introducing new notification templates (Email/SMS) and migrating customers to on-premises solutions, a data migration is currently required for existing tenants. However, with the removal of data migration support in upcoming Identity Server (IS) releases, we need to address this issue to ensure smooth template integration without relying on data migration.

Proposed Fix Previously, email templates for all tenants were stored in the database, necessitating data migration whenever new templates were added. To avoid this, we propose skipping the addition of default email templates during tenant creation, thereby preventing the storage of these templates in the database. Under the new approach, the database will only store modified email templates. If a template is not found in the database, the system will automatically retrieve it from the default set, ensuring seamless access to templates without requiring migration.

Current Flow

Proposed Approach