[Investigation] Handle license downgrades for custom work item types

We need to decide on what happens when a customer downgrades to a plan that does not support custom types.

For types that are renamed, we can display the original system-defined name / icon.

For new types, should they be hidden? Or fallback to issue type?

For epics today, they become hidden when the user downgrades.

Decision

On license downgrade, we keep all existing custom types and their configurations accessible for reading, but block creating new custom types and modifying existing ones. This avoids destructive actions and data loss while clearly communicating reduced functionality. We enforce a limit of 30 custom types per namespace/organization for Premium tier. Future tier differentiation (e.g., hierarchy depth limits) will follow the same keep-but-restrict-mutations strategy.