Remove the CollectionHandler field from router.Config struct and its
initialization in main.go. This field was never used - collections are
registered directly in handlers.SetupRoutes() where a CollectionHandler
is created locally.
Changes:
- Remove CollectionHandler field from internal/router/router.go Config
- Remove CollectionHandler: nil line from cmd/server/main.go
This cleans up dead code from the router refactoring. Collections
continue to work correctly as they are registered in SetupRoutes().
Related: Router refactoring completion