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>

renovate.json

@@ -10,5 +10,15 @@
       "matchPackageNames": ["pillow"],
       "allowedVersions": "<12.0.0"
     }
+  ],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "fileMatch": ["^\\.github/workflows/test\\.yml$"],
+      "matchStrings": [
+        "nextcloud_image:\\s*\"(?<depName>[^:]+):(?<currentValue>[^@]+)@(?<currentDigest>sha256:[a-f0-9]+)\""
+      ],
+      "datasourceTemplate": "docker"
+    }
   ]
 }