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>

third_party/astrolabe/tests/unit/phpunit.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="utf-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         bootstrap="bootstrap.php"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.5/phpunit.xsd"
+         colors="true"
+         failOnWarning="true"
+         failOnRisky="true"
+         cacheDirectory=".phpunit.cache">
+    <testsuite name="Astrolabe Unit Tests">
+        <directory suffix="Test.php">.</directory>
+    </testsuite>
+    <source>
+        <include>
+            <directory suffix=".php">../../lib</directory>
+        </include>
+    </source>
+</phpunit>