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.schema.json

@@ -46,8 +46,8 @@
               "type": "boolean"
             },
             "rejectUnauthorized": {
-              "description": "Passed to the app as SMTP_REJECT_UNAUTHORIZED. Read by the chart but deliberately without a default in values.yaml - see the note in the PR that introduced this schema.",
-              "type": ["boolean", "string", "null"]
+              "description": "Verify the mail server's TLS certificate; passed to the app as SMTP_REJECT_UNAUTHORIZED. Defaults to true (issue #109). Set false only for a mail server with a self-signed certificate. Strings are rejected: the env var is rendered from this value, and a string like \"yes\" would reach the app as a value it does not read as a boolean. Null means the chart default (true), as elsewhere in this chart (issue #99).",
+              "type": ["boolean", "null"]
             }
           }
         },