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>

internal/web/templates/backup.html

@@ -11,7 +11,8 @@
   <p class="muted"><strong>The backup file is a secret</strong> (it contains
   private keys and credentials). Store and transfer it securely and delete it
   once the restore succeeds. Encrypting it below is the simplest way to do that:
-  the download is then a <code>.spbk</code> file that only the password opens.</p>
+  the download is then a <code>.spbk</code> file (SelfPost backup) that only
+  the password opens.</p>
   {{if .BackupErr}}<p class="error">{{.BackupErr}}</p>{{end}}
   <form method="post" action="/backup">
     {{template "encryptfields" .}}
@@ -22,9 +23,10 @@
 <div class="card">
   <h2>Import a domain</h2>
   <p class="muted">Move a single domain here from another SelfPost instance using
-  a domain export file (from that domain's page). Its DKIM key and application
-  passwords come across, so the published DNS record needs no change. The export
-  file is a secret, like a full backup.</p>
+  a domain export file (from that domain's page) — plain <code>.json</code> or
+  encrypted <code>.spde</code> (SelfPost domain export). Its DKIM key and
+  application passwords come across, so the published DNS record needs no
+  change. The export file is a secret, like a full backup.</p>
   {{if .ImportErr}}<p class="error">{{.ImportErr}}</p>{{end}}
   <form method="post" action="/domains/import" enctype="multipart/form-data">
     <label for="importfile">Domain export file</label>

internal/web/templates/domain_detail.html

@@ -299,7 +299,7 @@
   <p class="muted"><strong>The export file is a secret</strong> — it contains the
   private DKIM key and application passwords. Transfer it securely and delete it
   after the import, or encrypt it below and move a <code>.spde</code> file
-  instead; the import form asks for the password.</p>
+  (SelfPost domain export) instead; the import form asks for the password.</p>
   {{if .ExportErr}}<p class="error">{{.ExportErr}}</p>{{end}}
   <form method="post" action="/domains/{{.Domain.ID}}/export">
     {{template "encryptfields" .}}

internal/web/templates/encrypt_fields.html

@@ -1,9 +1,10 @@
 {{/* Password fields shared by the full-backup and domain-export forms. The
      checkbox is the switch: unticked, the download keeps its historic plain
      form (.tar.gz / .json); ticked, the file is sealed in a password-encrypted
-     envelope (.spbk / .spde). The fields start hidden and are revealed by
-     panel.js — with JavaScript blocked they are simply always visible, and the
-     server still decides from the checkbox alone. */}}
+     envelope (.spbk = SelfPost backup, .spde = SelfPost domain export). The
+     fields start hidden and are revealed by panel.js — with JavaScript blocked
+     they are simply always visible, and the server still decides from the
+     checkbox alone. */}}
 {{define "encryptfields"}}
 <div class="encrypt">
   <label class="check">