Rename internal/{checker,service,provider,notification} to
internal/{dnschecker,serviceanalyzer,providerregistry,notifier} so
imports no longer collide with the same-named usecase subpackages and
no longer require import aliases to disambiguate.
Analogous to internal/provider, extract the service registry (Svc,
RegisterService, FindService, ListServices, OrderedServices, FindSubService,
RegisterSubServices) and the zone analyzer (ServiceAnalyzer, Analyzer,
AnalyzeZone) from services/ into a new internal/service package.
- Moved usecases into internal/usecase
- Renamed internal/utils to internal/helpers for better naming semantics
- Clarified separation between domain model, usecases, adapters, and web/API
