Nextcloud Talk integration cleanup: unify secrets and signaling config

- Replace inline get_app_conf secrets in env.j2 with dedicated vars (TURN, signaling, internal)
- Correctly model signaling_servers as object {servers, secret} in spreed.yml
- Use UDP stun_turn port instead of TLS for transport=udp
- Add fallback logic for standalone Coturn role in main.yml
- Remove obsolete Greenlight section from BBB override

Ref: https://chatgpt.com/share/68d74e25-c068-800f-ae20-d0e34ac8ee12

roles/web-app-bigbluebutton/templates/docker-compose.override.yml.j2

@@ -29,11 +29,6 @@ services:
       --cert=${COTURN_TLS_CERT_PATH}
       --pkey=${COTURN_TLS_KEY_PATH}
 {% endif %}
-{% if BBB_GREENLIGHT_ENABLED | bool %}
-  greenlight:
-{% set container_port = 3000 %}
-{% include 'roles/docker-container/templates/healthcheck/nc.yml.j2' %}
-{% endif %}
 {% if BBB_COLLABORA_ENABLED | bool %}
   bbb-web:
     depends_on: