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_recovery.go

@@ -0,0 +1,50 @@
+package server
+
+import (
+	"log/slog"
+	"net/http"
+	"runtime/debug"
+)
+
+// RecoveryOption configures the Recovery middleware.
+type RecoveryOption func(*recoveryOptions)
+
+type recoveryOptions struct {
+	logger *slog.Logger
+}
+
+// WithRecoveryLogger sets the logger for the Recovery middleware.
+// If not set, panics are recovered silently (500 is still returned).
+func WithRecoveryLogger(l *slog.Logger) RecoveryOption {
+	return func(o *recoveryOptions) { o.logger = l }
+}
+
+// Recovery returns a middleware that recovers from panics in downstream
+// handlers. A recovered panic results in a 500 Internal Server Error
+// response and is logged (if a logger is configured) with the stack trace.
+func Recovery(opts ...RecoveryOption) Middleware {
+	o := &recoveryOptions{}
+	for _, opt := range opts {
+		opt(o)
+	}
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			defer func() {
+				if v := recover(); v != nil {
+					if o.logger != nil {
+						o.logger.LogAttrs(r.Context(), slog.LevelError, "panic recovered",
+							slog.Any("panic", v),
+							slog.String("stack", string(debug.Stack())),
+							slog.String("method", r.Method),
+							slog.String("path", r.URL.Path),
+							slog.String("request_id", RequestIDFromContext(r.Context())),
+						)
+					}
+					http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+				}
+			}()
+			next.ServeHTTP(w, r)
+		})
+	}
+}