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

cmd/greet/main.go

@@ -8,10 +8,11 @@ import (
 )
 
 func main() {
+	service := greet.NewService()
 	name := ""
 	if len(os.Args) > 1 {
 		name = os.Args[1]
 	}
 	
-	fmt.Println(greet.Greet(name))
+	fmt.Println(service.Greet(name))
 }