feat: Hide secrets on api and channels

docs/vantage/notification-channels.md

@@ -64,6 +64,15 @@ Posts the alert as message content.
 
 Port `465` uses implicit TLS; anything else uses STARTTLS.
 
+### Credentials are never read back
+
+The SMTP `password`, the Telegram `token` and the webhook, Slack and Discord
+`url`s come back from `GET /api/channels` as `••••••••` — a webhook URL is the
+authorisation to post to that channel, so it is treated as a credential like
+the rest. Writing that value back unchanged keeps the stored one, which is what
+lets you rename a channel without retyping its password. Anything else you send
+is written as given, so clearing the field clears the credential.
+
 Alert emails look like the rest of the mail Vantage sends you.
 
 ## The message