feat(ci): add Nextcloud version matrix (NC 31, 32, 33)

- Add cross-product matrix (3 versions x 4 auth modes = 12 CI jobs)
- Parameterize Nextcloud image in docker-compose.yml via NEXTCLOUD_IMAGE env var
- Pin NC 31.0.8, 32.0.6, 33.0.0 with SHA digests in workflow
- Add Renovate customManagers to auto-update NC images in workflow
- Fix Astrolabe install hook to prefer volume mount over app store
- Bump Astrolabe submodule to support NC 33 (max-version 31→33)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -23,7 +23,7 @@ services:
     restart: always
 
   app:
-    image: docker.io/library/nextcloud:32.0.6@sha256:5c4e09f72f096cd68379a8ae69f71e61d13da5a07430fc4a17c702a14e6a4267
+    image: ${NEXTCLOUD_IMAGE:-docker.io/library/nextcloud:32.0.6@sha256:5c4e09f72f096cd68379a8ae69f71e61d13da5a07430fc4a17c702a14e6a4267}
     restart: always
     ports:
       - 127.0.0.1:8080:80