fix(notification): save smtp sender without reentering password

Preserve the saved SMTP password when connection settings are unchanged and keep it out of admin forms.

Ref: IT-1601

docs/email-template-definition-api.md

@@ -75,6 +75,8 @@ The service/catalog lookup, validation, revision check, explicit removals, trans
 
 In the admin channel editor, **Send Test Email** tests the form's current values without saving them. An explicitly entered password takes precedence. When the password field is blank for an existing channel, the Web server reads its saved password only if host, port, username, and SSL/TLS mode still match the saved SMTP connection. Changed connection settings require an explicit password. The saved password is not placed in the page or test response. A new channel keeps the ad-hoc test behavior, including unauthenticated SMTP when username and password are empty.
 
+Saving an existing channel follows the same password rule: a blank password keeps the saved value when host, port, username, and SSL/TLS mode are unchanged, so sender name/address and limit edits do not require re-entering it. A changed SMTP connection needs an explicitly entered password. Creating a channel in the admin editor still requires a password. Validation errors redisplay the editor without rendering the saved or submitted password.
+
 - `GET /api/v1/email-channels/all` enumerates **all** channels, including channels whose exact `serviceName` has no service row.
 - Existing `GET /api/v1/email-channels?serviceName={exactName}`, GET by ID, POST, PUT by ID and DELETE by ID remain available. Create returns 201/Location and its existing GUID response; update/delete return 200/Unit.
 - Create supports SMTP (`channelType: 1`). Request/response `settings` explicitly maps `host`, `port`, `username`, `password`, `useSsl`, `fromEmail`, and `fromName`; reads no longer lose derived SMTP fields through an abstract JSON type. Host/fromEmail and a valid port are required. The optional empty username/password support unauthenticated development SMTP.