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

database/migrations/0002_saas_identity_tenants.sql

@@ -34,9 +34,6 @@ CREATE TABLE IF NOT EXISTS users (
   display_name VARCHAR(255) NOT NULL,
   password_hash VARCHAR(255) NULL,
   status VARCHAR(30) NOT NULL DEFAULT 'active',
-  email_verified_at DATETIME NULL,
-  mfa_enabled TINYINT(1) NOT NULL DEFAULT 0,
-  disabled_at DATETIME NULL,
   last_login_at DATETIME NULL,
   created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
   updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
@@ -79,9 +76,7 @@ CREATE TABLE IF NOT EXISTS participants (
   updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   UNIQUE KEY uq_participants_tenant_legacy (tenant_id, legacy_mitarbeiter_id),
   UNIQUE KEY uq_participants_tenant_email_norm (tenant_id, email_norm),
-  UNIQUE KEY uq_participants_tenant_user (tenant_id, user_id),
   KEY idx_participants_tenant_active_name (tenant_id, active, display_name),
-  KEY idx_participants_tenant_paypal_name (tenant_id, paypal_name),
   KEY idx_participants_user (user_id),
   CONSTRAINT fk_participants_tenant
     FOREIGN KEY (tenant_id) REFERENCES tenants(id)