feat: Implement comprehensive architectural guidelines and clean code standards

- Establish feature-based architecture with clear separation between Logic, Data, and Shared layers
- Define directory structure conventions for controllers, entities, DTOs, managers, mappers, models, processors, providers, repositories, and services
- Implement Symfony 7.4+ compliance with proper Request handling (UI layer only)
- Enforce PHP 8+ attributes usage instead of annotations
- Set up proper DTO usage in UseCases with immutable properties
- Configure repository pattern with interfaces in Logic layer and implementations in Data layer
- Implement strict separation of concerns with UI layer handling Request objects exclusively
- Define clear naming conventions (PascalCase for feature names)
- Add shared components directory for reusable elements across features
- Establish HTTP status code constants usage instead of magic numbers
- Configure proper file placement for controllers in UI/Frontend, UI/Api, and UI/CLI subdirectories
- Enforce modern PHP practices with explicit return types and strict typing

This commit lays the foundation for a maintainable, scalable Symfony 7.4+ application following clean architecture principles and modern coding standards.

.continue/rules/ui-layer-separation.md

@@ -0,0 +1,9 @@
+---
+globs: "**/*.php"
+description: Diese Regel stellt sicher, dass die UI-Schicht klar in Frontend,
+  API und CLI unterteilt ist, was die Wartbarkeit und Skalierbarkeit des
+  Projekts verbessert.
+alwaysApply: true
+---
+
+Alle Dateien in der UI-Schicht müssen in feature-spezifische Unterverzeichnisse unter /src/UI platziert werden: /src/UI/Frontend, /src/UI/Api oder /src/UI/CLI.