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/middleware_logging.go

@@ -0,0 +1,39 @@
+package server
+
+import (
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+// Logging returns a middleware that logs each request's method, path,
+// status code, duration, and request ID using the provided structured logger.
+func Logging(logger *slog.Logger) Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			start := time.Now()
+
+			sw := &statusWriter{ResponseWriter: w, status: http.StatusOK}
+			next.ServeHTTP(sw, r)
+
+			duration := time.Since(start)
+			attrs := []slog.Attr{
+				slog.String("method", r.Method),
+				slog.String("path", r.URL.Path),
+				slog.Int("status", sw.status),
+				slog.Duration("duration", duration),
+			}
+
+			if id := RequestIDFromContext(r.Context()); id != "" {
+				attrs = append(attrs, slog.String("request_id", id))
+			}
+
+			level := slog.LevelInfo
+			if sw.status >= http.StatusInternalServerError {
+				level = slog.LevelError
+			}
+
+			logger.LogAttrs(r.Context(), level, "request completed", attrs...)
+		})
+	}
+}