web-app-mediawiki: installer-driven bootstrap, DB readiness, idempotent admin; drop LocalSettings bind-mount

Tasks:
- Enable docker_compose_flush_handlers=true so services come up immediately.
- Add DB readiness guard via maintenance/sql.php (SELECT 1).
- Run maintenance/install.php on empty schema with robust changed_when/failed_when (merge stdout+stderr); keep secrets hidden.
- Run maintenance/update.php for migrations with neutral changed_when unless work is done.
- Make admin creation idempotent: tolerate 'already exists' and 'Account exists', keep async+no_log.

Config changes:
- Remove LocalSettings.php template and its host bind-mount from compose.
- Drop MediaWiki settings path variables and META namespace variable (unused after switch).

Result: First boot is fully automated (schema + admin), subsequent runs are cleanly idempotent.

Ref: ChatGPT conversation (Aug 28, 2025, Europe/Berlin) — https://chatgpt.com/share/68b0d2e1-9bc0-800f-81a5-db03ce0b81e3.

roles/web-app-mediawiki/templates/docker-compose.yml.j2

@@ -5,7 +5,6 @@
     image: "{{ MEDIAWIKI_IMAGE }}:{{ MEDIAWIKI_VERSION }}"
     volumes:
       - "data:/var/www/html/"
-      - "{{ MEDIAWIKI_SETTINGS_HOST_PATH }}:{{ MEDIAWIKI_SETTINGS_DOCK_PATH }}:ro"
     ports:
       - "127.0.0.1:{{ ports.localhost.http[application_id] }}:{{ container_port }}"
 {% include 'roles/docker-container/templates/healthcheck/curl.yml.j2' %}