Translation Manager: fix SiteIds cache growth (17.9.1), forward-port to v18 (18.3.1)

Fixed the culture mismatch (PR #65) and made OrderSetsAndSites/AddSiteDetails clone the set before mutating it (PR #66), so per-request response shaping can no longer write back into the cache
Released v17.9.1 to NuGet; published customer-facing notes on the companion Issues repo
Forward-ported both fixes into v18/main: merged v17/main into v18/main (PR #67), resolving conflicts in CHANGELOG.md and SetsControllerBase.cs
Released v18.3.1 to NuGet; published matching customer-facing notes on the companion Issues repo