Refactor to JSON API with Chi router and interface-based design

- Add pkg/greet/api_v1.go with ApiV1Greet interface and handler
- Implement Greeter interface for dependency injection
- Return JSON responses with proper Content-Type headers
- Move greet service instantiation to server package
- Separate v1 route registration into registerApiV1Routes function
- Health endpoint at /api level, greet endpoints at /api/v1/greet
- Simplify server entrypoint by removing external dependencies

pkg/greet/greet.go

@@ -1,8 +1,15 @@
 package greet
 
+type Service struct{}
+
+func NewService() *Service {
+	return &Service{}
+}
+
 // Greet returns a greeting message for the given name.
 // If name is empty, it defaults to "world".
-func Greet(name string) string {
+// Implements the Greeter interface.
+func (s *Service) Greet(name string) string {
 	if name == "" {
 		return "Hello world!"
 	}