test(astrolabe): install app from Nextcloud app store; bump submodule to v0.16.6

Stop mounting the vendored astrolabe submodule into the Nextcloud `app`
container by default (comment out the /opt/apps/astrolabe bind mount). With
the mount absent, the post-installation hook (20-install-astrolabe-app.sh)
falls through to `occ app:install astrolabe` + `app:enable`, so the dev/CI
stack now exercises the published app-store package rather than a locally
built dev copy. This catches packaging issues (e.g. missing built assets in
the released app) that a source build would mask.

Bump the third_party/astrolabe submodule to v0.16.6, which includes the
background-indexing re-login fix (astrolabe#93).

The dev mount and the CI "Build Astrolabe app" step are retained (commented
mount can be re-enabled locally) so developers can still iterate against the
vendored source on demand.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -36,7 +36,7 @@ services:
       # Mount OIDC development directory outside /var/www/html to avoid rsync conflicts
       # The post-installation hook will register /opt/apps as an additional app directory
       #- ./third_party:/opt/apps:ro
-      - ./third_party/astrolabe:/opt/apps/astrolabe:ro
+      #- ./third_party/astrolabe:/opt/apps/astrolabe:ro
       #- ./third_party/oidc:/opt/apps/oidc:ro
     environment:
       - NEXTCLOUD_TRUSTED_DOMAINS=app