Reusable GitHub Action per repo

Chose a reusable GitHub Action installed per repository over a central shared dispatcher because Burak Yigit Kaya wants newsletter delivery to be per-repo, with each repo supplying its own Loops API key and mailing-list settings as repo secrets/inputs. Rejected a shared multi-tenant service as the default because it looks reusable, but it centralizes credentials and state that Burak Yigit Kaya explicitly wants each repository to own.