feat(admin): one password change reaches every instance

Best-effort by design: refusing the change because one instance was
unreachable would leave the customer holding the password they were trying
to replace. A failure is flagged and hqsync repairs it.

Also corrects two pieces of copy this makes false — CreateInstance's doc
comment and the instance-ready email both claimed the two passwords
diverge.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/internal/mail/mail.go

@@ -178,7 +178,7 @@ func SendInstanceReady(to, instanceName, loginURL string, expires time.Time) err
 	body += fmt.Sprintf(
 		"Your Free licence runs until %s. We will email you before then so you can renew it in one click.\n\n"+
 			"Sign in with the same email address and password you use for your Vantage account. "+
-			"Changing one does not change the other.\n",
+			"Changing your Vantage HQ password changes it here too.\n",
 		expires.Format("2 January 2006"))
 	return send(to, instanceName+" is ready", body)
 }