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/lib/hooks/usePermissions.ts

@@ -25,5 +25,6 @@ export function usePermissions() {
     isLoading,
     userId: data?.user_id ?? null,
     tenantId: data?.tenant_id ?? null,
+    isDemo: data?.is_demo ?? false,
   };
 }