Reflect any Origin so token-authenticated clients (Access Token V2 / PAT)
can call the API cross-origin, but emit Access-Control-Allow-Credentials
only for trusted origins (same host / configured InstanceURL). This keeps
the SameSite=Lax refresh cookie unreadable by untrusted (incl. same-site
subdomain) origins. Origin: null is not reflected.
Note for operators: cross-origin token access is now open by default; if
you front memos with a caching proxy, ensure it honors `Vary: Origin`.
