Fix type hints regressions (#4855)

Fix regressions from https://github.com/FreshRSS/FreshRSS/pull/4561

Example:

```
PHP Fatal error:  Uncaught TypeError: Argument 1 passed to checkToken() must be an instance of FreshRSS_UserConfiguration, instance of Minz_Configuration given, called in /var/www/FreshRSS/p/api/greader.php on line 1091 and defined in /var/www/FreshRSS/p/api/greader.php:223
Stack trace:
#0 /var/www/FreshRSS/p/api/greader.php(1091): checkToken()
#1 {main}
  thrown in /var/www/FreshRSS/p/api/greader.php on line 223
```

Improvement of https://github.com/FreshRSS/FreshRSS/pull/4110

1 files changed, 5 insertions, 1 deletions

diff --git a/app/Models/SystemConfiguration.php b/app/Models/SystemConfiguration.php
index ec5960c0e..9fc79969d 100644
--- a/app/Models/SystemConfiguration.php
+++ b/app/Models/SystemConfiguration.php
@@ -25,6 +25,10 @@
  * @property string $unsafe_autologin_enabled
  * @property-read array<string> $trusted_sources
  */
-class FreshRSS_SystemConfiguration extends Minz_Configuration {
+final class FreshRSS_SystemConfiguration extends Minz_Configuration {
 
+	public static function init($config_filename, $default_filename = null): FreshRSS_SystemConfiguration {
+		parent::register('system', $config_filename, $default_filename);
+		return parent::get('system');
+	}
 }