✨ feat(server): add /api/healthz endpoint with rich health info

Adds a Kubernetes-style healthz endpoint returning status, version,
uptime_seconds and timestamp. Non-breaking — /api/health is preserved.

- New route: GET /api/healthz
- New handler: handleHealthz with HealthzResponse struct
- New unit test: pkg/server/healthz_test.go (passes locally)
- New BDD scenario: features/health/health.feature
- BDD steps: pkg/bdd/steps/health_steps.go, common_steps.go

Note: BDD tests require Postgres and will be validated by CI.

🤖 Co-Authored-By: Mistral Vibe (devstral-2 / mistral-medium-3.5)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/server/server.go

@@ -64,6 +64,7 @@ type Server struct {
 	validator      *validation.Validator
 	userRepo       user.UserRepository
 	userService    user.UserService
+	startedAt      time.Time
 }
 
 func NewServer(cfg *config.Config, readyCtx context.Context) *Server {
@@ -89,6 +90,7 @@ func NewServer(cfg *config.Config, readyCtx context.Context) *Server {
 		validator:   validator,
 		userRepo:    userRepo,
 		userService: userService,
+		startedAt:   time.Now(),
 	}
 	s.setupRoutes()
 	return s
@@ -137,6 +139,9 @@ func (s *Server) setupRoutes() {
 	// Version endpoint at root level
 	s.router.Get("/api/version", s.handleVersion)
 
+	// Kubernetes-style health endpoint at root level
+	s.router.Get("/api/healthz", s.handleHealthz)
+
 	// API routes
 	s.router.Route("/api/v1", func(r chi.Router) {
 		r.Use(s.getAllMiddlewares()...)
@@ -358,6 +363,34 @@ func (s *Server) handleVersion(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// HealthzResponse represents the Kubernetes-style health check response
+type HealthzResponse struct {
+	Status        string    `json:"status"`
+	Version       string    `json:"version"`
+	UptimeSeconds int64     `json:"uptime_seconds"`
+	Timestamp     time.Time `json:"timestamp"`
+}
+
+// handleHealthz godoc
+//
+//	@Summary		Kubernetes-style health check
+//	@Description	Returns rich health info for liveness/readiness probes
+//	@Tags			System/Health
+//	@Produce		json
+//	@Success		200	{object}	HealthzResponse
+//	@Router			/healthz [get]
+func (s *Server) handleHealthz(w http.ResponseWriter, r *http.Request) {
+	log.Trace().Msg("Healthz check requested")
+	resp := HealthzResponse{
+		Status:        "healthy",
+		Version:       version.Version,
+		UptimeSeconds: int64(time.Since(s.startedAt).Seconds()),
+		Timestamp:     time.Now().UTC(),
+	}
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(resp)
+}
+
 func (s *Server) Router() http.Handler {
 	return s.router
 }