M0: Skelett + Auth-Grundgerüst (Keycloak-OIDC + API-Token)

- Symfony 7.4 LTS, PHP 8.4, FrankenPHP, SQLite (3-Layer src/UI|Logic|Data)
- Auth: manueller OidcClient (Authorization-Code) im UI-Layer, kein
  Security-Bundle/league; Session-Gate via SessionAuthSubscriber
- API-Guard via statischem X-Api-Token (ApiTokenSubscriber); ExceptionSubscriber
- UserEntity (Auto-Provision) + Init-Migration
- Docker (php:8.4-frankenphp) + compose (essen.ljh31.de, traefik-net external)
- SmokeTests (7) grün: Redirect-Fälle, API-Token, Auth-Render, Logout

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Logic/Common/Exception/ValidationException.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Logic\Common\Exception;
+
+final class ValidationException extends DomainException
+{
+    /**
+     * @var array<string, list<string>> Feld → Liste von Fehlermeldungen
+     */
+    public function __construct(
+        private readonly array $errors = [],
+    ) {
+        parent::__construct('Validierungsfehler.');
+    }
+
+    /** @param array<string, list<string>> $errors */
+    public static function fieldErrors(array $errors): self
+    {
+        return new self($errors);
+    }
+
+    /** @return array<string, list<string>> */
+    public function getErrors(): array
+    {
+        return $this->errors;
+    }
+
+    public function getHttpStatusCode(): int
+    {
+        return 422;
+    }
+}