refactor(healthz): скрыть Server за интерфейсом

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

cmd/synapse-backupper/run.go

@@ -29,7 +29,7 @@ var (
 	) {
 		return cron.NewCronScheduler(expr, job)
 	}
-	newHealthz = func(port int) (*healthz.Server, error) {
+	newHealthz = func(port int) (healthz.Server, error) {
 		return healthz.New(port)
 	}
 	runOnceFunc = backup.RunOnce

pkg/adapters/healthz/healthz.go

@@ -9,23 +9,34 @@ import (
 	"time"
 )
 
-// Server is a minimal HTTP health check server.
+// Server is the public interface of the minimal HTTP health check server.
-type Server struct {
+type Server interface {
+	// Addr returns the bound network address (e.g. "127.0.0.1:8080").
+	Addr() string
+	// Start begins serving HTTP requests. It blocks until Stop is called.
+	Start() error
+	// Stop initiates graceful shutdown. After Stop is called the /healthz
+	// endpoint returns 503 while in-flight requests complete.
+	Stop(ctx context.Context) error
+}
+
+// server is the private implementation of Server.
+type server struct {
 	listener     net.Listener
-	server       *http.Server
+	httpServer   *http.Server
 	shuttingDown atomic.Bool
 }
 
 // New creates a health check server listening on the given port.
 // Passing port 0 binds to an available ephemeral port.
-func New(port int) (*Server, error) {
+func New(port int) (Server, error) {
 	listener, err := net.Listen("tcp", fmt.Sprintf("127.0.0.1:%d", port))
 	if err != nil {
 		return nil, fmt.Errorf("failed to create listener: %w", err)
 	}
 
-	s := &Server{listener: listener}
+	s := &server{listener: listener}
-	s.server = &http.Server{
+	s.httpServer = &http.Server{
 		Handler:           http.HandlerFunc(s.handleHealthz),
 		ReadHeaderTimeout: 5 * time.Second,
 	}
@@ -33,7 +44,7 @@ func New(port int) (*Server, error) {
 }
 
 // Addr returns the bound network address (e.g. "127.0.0.1:8080").
-func (s *Server) Addr() string {
+func (s *server) Addr() string {
 	if s.listener == nil {
 		return ""
 	}
@@ -41,21 +52,21 @@ func (s *Server) Addr() string {
 }
 
 // Start begins serving HTTP requests. It blocks until Stop is called.
-func (s *Server) Start() error {
+func (s *server) Start() error {
-	return s.server.Serve(s.listener)
+	return s.httpServer.Serve(s.listener)
 }
 
 // Stop initiates graceful shutdown. After Stop is called the /healthz
 // endpoint returns 503 while in-flight requests complete.
-func (s *Server) Stop(ctx context.Context) error {
+func (s *server) Stop(ctx context.Context) error {
 	s.shuttingDown.Store(true)
 	// Grace period so that health checks can observe the 503 state
 	// before the listener is closed.
 	time.Sleep(100 * time.Millisecond)
-	return s.server.Shutdown(ctx)
+	return s.httpServer.Shutdown(ctx)
 }
 
-func (s *Server) handleHealthz(w http.ResponseWriter, r *http.Request) {
+func (s *server) handleHealthz(w http.ResponseWriter, r *http.Request) {
 	if r.URL.Path != "/healthz" {
 		http.NotFound(w, r)
 		return