docs(astrolabe): add config docs and unit tests for internal URL

Address PR #487 reviewer feedback:

- Add documentation for `astrolabe_internal_url` config option
- Add unit tests for `IdpTokenRefresher::getNextcloudBaseUrl()`
- Fix CI workflow paths (astroglobe -> astrolabe)
- Add PHPUnit job to CI workflow for PHP 8.1, 8.2, 8.3
- Remove obsolete ApiTest that tested non-existent method

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/configuration.md

@@ -531,6 +531,28 @@ docker-compose up
 
 ---
 
+## Astrolabe Internal URL
+
+The Astrolabe Nextcloud app may need to make internal HTTP requests to the local web server (e.g., for OAuth token refresh). By default, it uses `http://localhost` which works for standard Docker containers where PHP and Apache run together.
+
+| Variable | Description | Default |
+|----------|-------------|---------|
+| `astrolabe_internal_url` | Internal URL for server-to-server requests within container | `http://localhost` |
+
+**When to configure:**
+- Custom container setups where the internal web server is not on `localhost:80`
+- Kubernetes deployments with service discovery
+- Multi-container setups with separate web server containers
+
+**Example (Nextcloud config.php):**
+```php
+'astrolabe_internal_url' => 'http://web-server.internal:8080',
+```
+
+**Note:** This is for internal PHP-to-Apache requests, NOT for external client URLs. The default (`http://localhost`) works for standard Docker containers where PHP and Apache run together.
+
+---
+
 ## Loading Environment Variables
 
 After creating your `.env` file, load the environment variables: