feat(ess_pro): deploy Element Server Suite Pro via K3s + Helm

Adds k3s and ess_pro roles to replace the planned Nextcloud Talk
stack. Integrates with existing Keycloak (OIDC), Garage (S3 media)
and OpenBao (secrets). Hostnames under digitalboard.ch.

roles/ess-pro/examples/site.yml

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: MIT-0
+---
+# Example play, mirroring the convention used by the other digitalboard.core
+# roles (digitalboard.core.bookstack, digitalboard.core.opnform, ...).
+#
+# Place the ess-pro role into your digitalboard.core collection alongside
+# the others, then reference it as `digitalboard.core.ess_pro`.
+
+- name: Deploy Element Server Suite Pro
+  hosts: ess_servers
+  become: true
+  roles:
+    - digitalboard.core.k3s
+    - digitalboard.core.ess_pro