refactor: migrate deprecated IConfig app-values to IAppConfig

[rubenvdlinde]

Summary

• Migrates IConfig::getAppValue in HealthController to the modern OCP\IAppConfig::getValueString typed accessor (NC26+ API)
• Adds IAppConfig constructor injection alongside the retained IConfig (still required for getSystemValueString)
• Behavior-preserving: same key, same default, same return type — no logic changes

Details

• File: lib/Controller/HealthController.php
• Call migrated: $this->config->getAppValue(appName: APP_ID, key: 'installed_version', default: '0.0.0') → $this->appConfig->getValueString(...)
• User-values (getUserValue/setUserValue/deleteUserValue in PreferencesController): left on IConfig — NC min-version is 28, OCP\IUserConfig requires NC≥31

Test plan

• php -l lib/Controller/HealthController.php — no syntax errors
• Gate-16 spec coverage: exit 0
• Health endpoint /api/health returns correct version string