add composer-unused, comprehensive docs, and project restructure

- Add icanhazstring/composer-unused as dev dependency for dependency hygiene checks
- Add German documentation (docs/) covering architecture, conventions, workflows, and developer checklists
- Add API layer (ApiAuth, ApiBootstrap, ApiResponse), audit, scheduler, custom fields, and SSO services
- Add Microsoft OIDC SSO, API token management, and user lifecycle features
- Add swagger-ui vendor integration and OpenAPI spec
- Add production Docker setup and bin/ scripts
- Update composer dependencies, config, templates, and frontend assets throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

phpstan.neon

@@ -1,13 +1,12 @@
 parameters:
     level: 5
-    phpVersion: 80500
+    phpVersion: 80499
     scanFiles:
         - web/index.php
     paths:
         - config
         - lib
         - pages
-        - templates
+        - tests
     fileExtensions:
         - php
-        - phtml