feat: add Session.cache_backend property

[dokson]

After 49566d7 the Session(cache=60) shorthand leaves no public handle to the backend (no clear(), no expires). This adds a read-only cache_backend property, None when caching is disabled. Named so it doesn't trip the hasattr(session, "cache") convention from #809.

Checklist

• I have manually reviewed the changes and fully understand the code.