docs: spell out .spbk and .spde extensions

SelfPost backup / SelfPost domain export — in the guide, security notes,
architecture, and Backup/Export panel copy.

Co-Authored-By: Composer <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

CHANGELOG.md

@@ -51,6 +51,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ### Changed
 
+- Document what `.spbk` and `.spde` stand for (SelfPost backup / SelfPost domain
+  export) in the operator guide, security notes, architecture, and the Backup /
+  Export panel copy.
 - Docs aligned with the code: setup URL is `/setup/<token>` (README and
   guide; local trial rewrites the printed `https://<hostname>/…` link to
   `http://127.0.0.1:8080/…`); domain import uses the file extension / magic