Add a Scoped RepositoryCache that memoizes deserialized XML lists,
keyed by absolute file path. Each XML file is loaded at most once per
SignalR circuit; Save invalidates the corresponding entry so
subsequent reads reload from disk.
BaseRepository gains a LoadList<T>(fileName) helper; each of the seven
repositories now defines Get*List() as a one-liner over that helper
and takes RepositoryCache via the constructor.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
