rallly: default smtp.rejectUnauthorized to true

The deployment renders SMTP_REJECT_UNAUTHORIZED from
rallly.smtp.rejectUnauthorized, but values.yaml never carried the key. With
SMTP enabled the variable therefore reached the app as an empty string --
a certificate check nobody had configured either way.

It now defaults to true, so the mail server's certificate is verified unless
the operator switches the check off deliberately. values.yaml explains when
that is needed (a mail server with a self-signed certificate), because the
symptom otherwise points at the mail server rather than at this value.

The env var is not rendered with "| default true": an explicit false is a
zero value and default would swallow it, silently re-enabling the check the
operator just switched off. A nil falls back to true instead, so the variable
can no longer reach the app empty. The schema now rejects strings.

Operationally inert: the rallly fleet bundle and the deployed release both
have smtp.enabled: false, so the variable is not rendered at all -- the
render with SMTP disabled is byte-identical to before.

Major, because the runtime behaviour of mail sending changes.

Fixes #109

rallly/values.yaml

@@ -16,6 +16,17 @@ rallly:
   replicas: 1
   smtp:
     enabled: false
+    # Verify the mail server's TLS certificate (passed to the app as
+    # SMTP_REJECT_UNAUTHORIZED). Leave this at true.
+    #
+    # Set it to false ONLY for a mail server with a self-signed certificate.
+    # Without that hint the symptom is misleading: the send fails with a
+    # certificate error naming the mail server, so the search starts at the
+    # mail server or its CA chain rather than at this value.
+    #
+    # false disables the certificate check entirely and exposes the SMTP
+    # session to a man-in-the-middle; prefer trusting the CA over setting it.
+    rejectUnauthorized: true
   allowedEmails: '*'
   supportEmail: null
   oidc: