Add production-ready HTTP server package with routing, health checks, and middleware

Introduces server/ sub-package as the server-side companion to the existing Client.
Includes Router (over http.ServeMux with groups and mounting), graceful shutdown with
signal handling, health endpoints (/healthz, /readyz), and built-in middlewares
(RequestID, Recovery, Logging). Zero external dependencies.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/health.go

@@ -0,0 +1,55 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// ReadinessChecker is a function that reports whether a dependency is ready.
+// Return nil if healthy, or an error describing the problem.
+type ReadinessChecker func() error
+
+// HealthHandler returns an http.Handler that exposes liveness and readiness
+// endpoints:
+//
+//   - GET /healthz — liveness check, always returns 200 OK
+//   - GET /readyz  — readiness check, returns 200 if all checkers pass, 503 otherwise
+func HealthHandler(checkers ...ReadinessChecker) http.Handler {
+	mux := http.NewServeMux()
+
+	mux.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_ = json.NewEncoder(w).Encode(healthResponse{Status: "ok"})
+	})
+
+	mux.HandleFunc("GET /readyz", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+
+		var errs []string
+		for _, check := range checkers {
+			if err := check(); err != nil {
+				errs = append(errs, err.Error())
+			}
+		}
+
+		if len(errs) > 0 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			_ = json.NewEncoder(w).Encode(healthResponse{
+				Status: "unavailable",
+				Errors: errs,
+			})
+			return
+		}
+
+		w.WriteHeader(http.StatusOK)
+		_ = json.NewEncoder(w).Encode(healthResponse{Status: "ok"})
+	})
+
+	return mux
+}
+
+type healthResponse struct {
+	Status string   `json:"status"`
+	Errors []string `json:"errors,omitempty"`
+}