Split the service layer into interface + impl (services/ + services/impl/)

Match the treasury/business convention: each service is a services/XService interface bound to a services/impl/XServiceImpl implementation, wired in Guice with bind(XService.class).to(XServiceImpl.class). 17 services split (14 *Service + Security/AdminBypass/PreviewHandler); ChestShopSign stays concrete (static-heavy sign util). Per-method javadoc moved onto the interfaces; impls carry @Override. Impl-internal tests (package-private/static helpers) moved to the services.impl test package.