Add GET /api/system/info and return SAP version from login

- capture Version + SessionTimeout from the Service Layer login
  response (SessionManager.loginInfo, survives logout) and return
  them from POST /api/session/login
- new GET /api/system/info: entity sets (service document),
  UserFieldsMD/UserTablesMD/UserObjectsMD customizations and
  CompanyService_GetAdminInfo; unreadable sections come back as
  { error } instead of failing the whole request
- OpenAPI: new System tag, examples for both endpoints
- tests for getSystemInfo section isolation and loginInfo retention

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -8,8 +8,9 @@ Součástí repozitáře je i malý AppFactory HTTP wrapper s endpointy:
 - `GET /health`
 - `GET /docs`
 - `GET /openapi.json`
-- `POST /api/session/login`
+- `POST /api/session/login` (vrací i verzi Service Layeru a session timeout)
 - `POST /api/session/logout`
+- `GET /api/system/info` (verze SAP, dostupné entity sety, UDF/UDT/UDO, admin info)
 - `GET /api/<resource>`
 - `GET /api/<resource>/all`
 - `GET /api/<resource>/{id}`