Simplify server config by consolidating BackendHost and BackendPort into
a single BindAddress field. The port is now solely controlled by FrontendPort.
Changes:
- Replace BackendHost/BackendPort with BindAddress in Config struct
- Add deprecation warning for BACKEND_HOST env var (use BIND_ADDRESS)
- Update connection timeout default from 45s to 60s
- Remove backendPort from SystemSettings and frontend types
- Update server.go to use cfg.BindAddress
- Update all tests to use new config field names
