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

database/migrations/0005_saas_tenant_resolution.sql

@@ -0,0 +1,16 @@
+CREATE TABLE IF NOT EXISTS tenant_domains (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  tenant_id INT NOT NULL,
+  domain VARCHAR(255) NOT NULL,
+  domain_norm VARCHAR(255) NOT NULL,
+  domain_type VARCHAR(30) NOT NULL DEFAULT 'custom',
+  status VARCHAR(30) NOT NULL DEFAULT 'active',
+  verified_at DATETIME NULL,
+  created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  UNIQUE KEY uq_tenant_domains_domain_norm (domain_norm),
+  KEY idx_tenant_domains_tenant_status (tenant_id, status),
+  CONSTRAINT fk_tenant_domains_tenant
+    FOREIGN KEY (tenant_id) REFERENCES tenants(id)
+    ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;