bbb: WIP—stabilize env/compose wiring & prep SFU override

Context: debugging mediasoup/WebRTC failures caused by empty/interpolated vars (EXTERNAL_IPv4, etc.).
- Normalize config/main.yml (ip6_enabled flag, enable greenlight/coturn) and tidy formatting.
- Extend vars/main.yml with BBB_* switches (IPv6, Greenlight, Coturn), TURN/Coturn cert paths.
- env.j2: wire secrets & toggles, guard IPv6 via BBB_IP6_ENABLED, switch LDAP/OIDC to role flags, add TURN/STUN, and general cleanup.
- tasks/main.yml: use BBB_* fact names, robust path joins, write docker-compose.override.yml, and notify compose on env/override changes.
- tasks/01_docker-compose.yml: reference new BBB_DOCKER_COMPOSE_* facts.
- Add templates/docker-compose.override.yml.j2 (placeholder for SFU overrides to avoid bad defaults during runs).
Rationale: make Compose brings deterministic (no empty ), paving the way to set MS_WEBRTC_LISTEN_IPS in override without risk.

Chat reference: debugging thread with GPT-5 Thinking on 2025-09-26 https://chatgpt.com/share/68d59d98-4388-800f-a627-07b6a603d0b2.

roles/web-app-bigbluebutton/tasks/01_docker-compose.yml

@@ -8,12 +8,12 @@
 
 - name: Slurp docker-compose.yml from remote host
   slurp:
-    src: "{{ docker_compose_file_origine }}"
+    src: "{{ BBB_DOCKER_COMPOSE_FILE_ORIGINE }}"
   register: compose_slurp
 
 - name: Transform docker-compose.yml with compose_mods
   copy:
     content: "{{ compose_slurp.content | b64decode | compose_mods(docker_repository_path, docker_compose.files.env) }}"
-    dest: "{{ docker_compose_file_final }}"
+    dest: "{{ BBB_DOCKER_COMPOSE_FILE_FINAL }}"
   notify:
     - docker compose up