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

login.php

@@ -0,0 +1,74 @@
+<?php
+
+require_once __DIR__ . '/functions.php';
+
+$pdo = app_db_pdo();
+$errors = [];
+$email = trim((string)($_POST['email'] ?? ''));
+$tenantSlug = trim((string)($_POST['tenant_slug'] ?? ''));
+
+if ($_SERVER['REQUEST_METHOD'] === 'POST') {
+    app_require_csrf();
+
+    $result = saas_authenticate(
+        $pdo,
+        $email,
+        (string)($_POST['password'] ?? ''),
+        $tenantSlug
+    );
+
+    if ($result['ok']) {
+        saas_session_login($result['identity']);
+        header('Location: konto.php');
+        exit;
+    }
+
+    $errors = $result['errors'];
+}
+
+include 'header.php';
+include 'headerline.php';
+include 'nav.php';
+?>
+
+<section id="banner">
+    <div class="content">
+        <h2>Login</h2>
+
+        <?php if (isset($_GET['logged_out'])): ?>
+            <div class="hint-box success"><p>Du wurdest abgemeldet.</p></div>
+        <?php endif; ?>
+
+        <?php if ($errors !== []): ?>
+            <div class="hint-box error">
+                <?php foreach ($errors as $error): ?>
+                    <p><?php echo saas_html($error); ?></p>
+                <?php endforeach; ?>
+            </div>
+        <?php endif; ?>
+
+        <form method="post" action="login.php">
+            <?php echo app_csrf_field(); ?>
+            <div class="row">
+                <div class="col-6 col-12-small">
+                    <label for="email">E-Mail</label>
+                    <input type="email" name="email" id="email" value="<?php echo saas_html($email); ?>" required>
+                </div>
+                <div class="col-6 col-12-small">
+                    <label for="password">Passwort</label>
+                    <input type="password" name="password" id="password" required>
+                </div>
+                <div class="col-6 col-12-small">
+                    <label for="tenant_slug">Kundenkuerzel</label>
+                    <input type="text" name="tenant_slug" id="tenant_slug" value="<?php echo saas_html($tenantSlug); ?>" placeholder="optional">
+                </div>
+            </div>
+            <ul class="actions">
+                <li><button type="submit">Einloggen</button></li>
+                <li><a href="register.php" class="button alt">Registrieren</a></li>
+            </ul>
+        </form>
+    </div>
+</section>
+
+<?php include 'footer.php'; ?>