feat: HL tenant setup + email domain auto-assignment

- Create pre-configured Hogan Lovells tenant with demo flag and
  auto_assign_domains: ["hoganlovells.com"]
- Add POST /api/tenants/auto-assign endpoint: checks email domain
  against tenant settings, auto-assigns user as associate if match
- Add AutoAssignByDomain to TenantService
- Update registration flow: after signup, check auto-assign before
  showing tenant creation form. Skip tenant creation if auto-assigned.
- Add DemoBanner component shown when tenant.settings.demo is true
- Extend GET /api/me to return is_demo flag from tenant settings

frontend/src/components/layout/DemoBanner.tsx

@@ -0,0 +1,17 @@
+"use client";
+
+import { usePermissions } from "@/lib/hooks/usePermissions";
+
+export function DemoBanner() {
+  const { isDemo, isLoading } = usePermissions();
+
+  if (isLoading || !isDemo) return null;
+
+  return (
+    <div className="flex items-center justify-center gap-2 bg-amber-50 border-b border-amber-200 px-4 py-2 text-sm text-amber-800">
+      <span className="font-medium">Demo-Modus</span>
+      <span className="text-amber-600">&mdash;</span>
+      <span>Keine echten Mandantendaten eingeben</span>
+    </div>
+  );
+}