M3 Tenant-Aufloesung per App-Session ergaenzen

- Mandantenauswahl fuer Benutzer mit mehreren Tenants bauen
- feste Tenant-Domains ohne Wildcard-Abhaengigkeit vorbereiten
- Login, Backfill, Smoke-Checks und M3-Dokumentation aktualisieren

mandant-auswahl.php

@@ -0,0 +1,74 @@
+<?php
+
+require_once __DIR__ . '/functions.php';
+
+$pdo = app_db_pdo();
+$pendingUserId = saas_pending_tenant_user_id();
+if ($pendingUserId === null) {
+    header('Location: login.php');
+    exit;
+}
+
+$errors = [];
+$memberships = saas_list_user_memberships($pdo, $pendingUserId);
+
+if ($_SERVER['REQUEST_METHOD'] === 'POST') {
+    app_require_csrf();
+
+    $tenantId = isset($_POST['tenant_id']) ? (int)$_POST['tenant_id'] : 0;
+    $identity = $tenantId > 0 ? saas_identity_for_user_tenant($pdo, $pendingUserId, $tenantId) : null;
+
+    if ($identity === null) {
+        $errors[] = 'Dieser Mandant ist fuer dein Konto nicht verfuegbar.';
+    } else {
+        saas_session_login($identity);
+        header('Location: konto.php');
+        exit;
+    }
+}
+
+include 'header.php';
+include 'headerline.php';
+include 'nav.php';
+?>
+
+<section id="banner">
+    <div class="content">
+        <h2>Mandant auswaehlen</h2>
+
+        <?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; ?>
+
+        <?php if ($memberships === []): ?>
+            <div class="hint-box error"><p>Fuer dieses Konto ist kein aktiver Mandant verfuegbar.</p></div>
+        <?php else: ?>
+            <form method="post" action="mandant-auswahl.php">
+                <?php echo app_csrf_field(); ?>
+                <div class="row">
+                    <div class="col-12">
+                        <label for="tenant_id">Mandant</label>
+                        <select name="tenant_id" id="tenant_id" required>
+                            <?php foreach ($memberships as $membership): ?>
+                                <option value="<?php echo saas_html($membership['tenant_id']); ?>">
+                                    <?php echo saas_html($membership['tenant_name']); ?>
+                                    (<?php echo saas_html($membership['role']); ?>)
+                                </option>
+                            <?php endforeach; ?>
+                        </select>
+                    </div>
+                </div>
+                <ul class="actions">
+                    <li><button type="submit">Weiter</button></li>
+                    <li><a href="logout.php" class="button alt">Abbrechen</a></li>
+                </ul>
+            </form>
+        <?php endif; ?>
+    </div>
+</section>
+
+<?php include 'footer.php'; ?>