M3 Registrierung und Login-Grundlage ergänzen

- SaaS-Auth-Helper und PDO-Datenbankzugriff einführen
- Registrierung, Login, Logout und Kontoansicht ergänzen
- Auth-Migration, Flow-Check und Smoke-Abdeckung aktualisieren

app/database.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+require_once __DIR__ . '/bootstrap.php';
+
+function app_db_pdo(): PDO
+{
+    static $pdo = null;
+
+    if ($pdo instanceof PDO) {
+        return $pdo;
+    }
+
+    $host = app_env('DB_HOST', '');
+    $database = app_env('DB_NAME', '');
+    $user = app_env('DB_USER', '');
+    $password = app_env('DB_PASS', '');
+    $port = app_env('DB_PORT', '3306');
+
+    if ($host === '' || $database === '' || $user === '') {
+        throw new RuntimeException('Database environment is incomplete.');
+    }
+
+    $dsn = sprintf('mysql:host=%s;port=%s;dbname=%s;charset=utf8mb4', $host, $port, $database);
+    $pdo = new PDO($dsn, $user, $password ?? '', [
+        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
+        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
+        PDO::ATTR_EMULATE_PREPARES => true,
+    ]);
+
+    return $pdo;
+}