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

scripts/backfill-default-tenant.php

@@ -89,6 +89,7 @@ $ownerEmailNorm = m3_email_norm(getenv('M3_DEFAULT_TENANT_OWNER_EMAIL') ?: null)
 $stats = [
     'applied_migrations' => count($applied),
     'participants' => 0,
+    'stale_participants_removed' => 0,
     'users' => 0,
     'memberships' => 0,
 ];
@@ -154,7 +155,8 @@ try {
            email = VALUES(email),
            email_norm = VALUES(email_norm),
            paypal_name = VALUES(paypal_name),
-           active = VALUES(active)'
+           active = VALUES(active),
+           legacy_mitarbeiter_id = VALUES(legacy_mitarbeiter_id)'
     );
     $upsertMembership = $pdo->prepare(
         'INSERT INTO tenant_memberships (tenant_id, user_id, role, status, joined_at)
@@ -206,6 +208,18 @@ try {
         $stats['memberships']++;
     }
 
+    $removeStaleParticipants = $pdo->prepare(
+        'DELETE p
+         FROM participants p
+         LEFT JOIN kl_Mitarbeiter m
+           ON m.MitarbeiterID = p.legacy_mitarbeiter_id
+         WHERE p.tenant_id = ?
+           AND p.legacy_mitarbeiter_id IS NOT NULL
+           AND m.MitarbeiterID IS NULL'
+    );
+    $removeStaleParticipants->execute([$tenantId]);
+    $stats['stale_participants_removed'] = $removeStaleParticipants->rowCount();
+
     $pdo->commit();
 } catch (Throwable $e) {
     $pdo->rollBack();
@@ -217,5 +231,6 @@ echo "M3 default tenant backfill complete.\n";
 echo "Tenant: {$tenantSlug}\n";
 echo "Applied migrations: {$stats['applied_migrations']}\n";
 echo "Participants mirrored: {$stats['participants']}\n";
+echo "Stale participants removed: {$stats['stale_participants_removed']}\n";
 echo "Users upserted: {$stats['users']}\n";
 echo "Memberships upserted: {$stats['memberships']}\n";