Centralized the From and Reply-To headers in Microwaveprop.Mailer.apply_defaults/1
so UserNotifier and EditNotifier can't drift. Drops the EMAIL_FROM env var
override — both values are hardcoded now.
Registered users can suggest edits to any contact's core fields
(callsigns, grids, band, mode, timestamp). Edits enter an admin
approval queue with field-by-field diff view. On approve, changes
are applied and enrichment re-enqueued if grids/band changed.
Users receive email notification on approve or reject.
Also updates dependabot.yml for mix ecosystem.
