SSR alert mail recipients maintainable in the site settings

New site setting vitec.ssr.alertEmails (Site Management, Settings, VITEC; one address per entry). vitec:ssr-health reads the recipients from there, invalid entries are skipped and logged, SSR_ALERT_EMAIL in the env file stays as fallback. The cockpit shows the recipients in the live band. Pre-filled with the address that was in the env file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/vitec/Configuration/Sets/Vitecset/settings.definitions.yaml

@@ -134,3 +134,10 @@ settings:
     category: VITEC
     type: bool
     default: true
+
+  vitec.ssr.alertEmails:
+    label: 'SSR alert mail recipients'
+    description: 'E-mail addresses that get the alert mails of the SSR health check: node down, node back, automatic fallback to the SPA. One address per entry; invalid addresses are skipped. Empty: the addresses from SSR_ALERT_EMAIL in var/vitec-ssr-purge.env are used.'
+    category: VITEC
+    type: stringlist
+    default: []