Refactor async execution handling across LDAP and Nextcloud roles

- Introduce global async configuration in group_vars/all/00_general.yml:
  - ASYNC_ENABLED (disabled in debug mode)
  - ASYNC_TIME (default 300s, omitted if async disabled)
  - ASYNC_POLL (0 for async fire-and-forget, 10 for sync mode)
- Replace hardcoded async/poll values with global vars in:
  - svc-db-openldap (03_users.yml, 04_update.yml)
  - web-app-mig (02_build_data.yml)
  - web-app-nextcloud (03_admin.yml, 04_system_config.yml, 05_plugin.yml,
    06_plugin_routines.yml, 07_plugin_enable_and_configure.yml)
- Guard changed_when and failed_when conditions to only evaluate in synchronous
  mode to avoid accessing undefined rc/stdout/stderr in async runs

  https://chatgpt.com/share/689cd8cc-7fbc-800f-bd06-a667561573bf

roles/web-app-mig/tasks/02_build_data.yml

@@ -3,8 +3,8 @@
     set -euo pipefail
     infinito build tree --no-signal --alarm-timeout 0 -s {{ mig_roles_meta_volume }}
     infinito build roles_list --no-signal --alarm-timeout 0 -o {{ mig_roles_meta_list }}
-  async: 3600
-  poll: 0
+  async: "{{ (3600 if ASYNC_ENABLED else omit) | default(omit) }}"
+  poll:  "{{ ASYNC_POLL if ASYNC_ENABLED else omit }}"
   register: mig_build_job
 
 - name: Fail if MIG build job did not start