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_test.go

@@ -2,7 +2,8 @@ package greet
 
 import "testing"
 
-func TestGreet(t *testing.T) {
+func TestService_Greet(t *testing.T) {
+	service := NewService()
 	tests := []struct {
 		name     string
 		expected string
@@ -15,7 +16,7 @@ func TestGreet(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			result := Greet(tt.name)
+			result := service.Greet(tt.name)
 			if result != tt.expected {
 				t.Errorf("Greet(%q) = %q, want %q", tt.name, result, tt.expected)
 			}