We should have validation at the model level before creating and updating to confirm that the extended release or extended release versions used are correct.

This should be done for repositories and templates, although users cannot write to the extended release fields for repositories.

Valid releases are "eus" and "e4s".

Valid versions are those listed in config.DistributionMinorVersions

As part of this ticket, we should also update the repository_parameters endpoint to return "eus" and "e4s" as the feature labels.

Acceptance Criteria:

• Unit test for repositories validation
• Creating a template with an invalid extended_release or extended_release_version returns a 400
• Repository parameters API returns the correct label but still filters by enable features. This may also require a small UI change.