uSync.Complete: Basic Auth push fix and first real NuGet release

uSync.Complete: found and fixed a bug where server-to-server push requests failed with 401 The request requires user authentication when the target had Umbraco's Basic Auth shared-secret bypass configured — the header was built with the wrong key/value pairing and got overwritten before the request went out
Migrated the nightly build feed from Azure Artifacts to the new shared nightly.jumoo.uk feed (R2/Sleet-based), switching to the mandated {version}-build.{increment} version scheme
Ran a full dry-run of the repo's NuGet release workflow for the first time — tagged a 17.4.1-alpha prerelease, confirmed all 12 packages published cleanly and installed into a fresh Umbraco 17 site with the backoffice UI intact — before cutting a real release
Released 17.4.1 to NuGet, published customer-facing notes on uSync.Complete.Issues, and forward-ported the fix and version bump to v18/main