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>

docker/php/Dockerfile

@@ -4,6 +4,7 @@ RUN apt-get update \
     && apt-get install -y --no-install-recommends \
         git \
         unzip \
+        libzip-dev \
         libmemcached-dev \
         zlib1g-dev \
         libssl-dev \
@@ -13,7 +14,7 @@ RUN apt-get update \
     && pecl install memcached \
     && docker-php-ext-enable memcached \
     && docker-php-ext-configure gd --with-jpeg --with-webp \
-    && docker-php-ext-install pdo_mysql mysqli gd \
+    && docker-php-ext-install pdo_mysql mysqli gd zip \
     && rm -rf /var/lib/apt/lists/*
 
 COPY --from=composer:2 /usr/bin/composer /usr/bin/composer