feat: funciones impuras HTTP — response, parse, logger, router, serve

Seis funciones de servidor HTTP con tests usando httptest:
- HTTPJSONResponse: escribe JSON con Content-Type y status code
- HTTPErrorResponse: escribe HTTPError como JSON estructurado
- HTTPParseBody: decode JSON con limite de bytes y campos estrictos
- HTTPLoggerMiddleware: loguea method/path/status/duration a io.Writer
- HTTPRouter: crea ServeMux con rutas Go 1.22+ (METHOD /path)
- HTTPServe: ListenAndServe con graceful shutdown por contexto

23 tests pasando, solo stdlib net/http.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/http_serve.go

@@ -0,0 +1,37 @@
+package infra
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// HTTPServe arranca un servidor HTTP en addr y realiza graceful shutdown cuando ctx se cancela.
+// Espera hasta 30 segundos para que las conexiones activas cierren antes de forzar el shutdown.
+func HTTPServe(addr string, handler http.Handler, ctx context.Context) error {
+	srv := &http.Server{
+		Addr:    addr,
+		Handler: handler,
+	}
+
+	errCh := make(chan error, 1)
+	go func() {
+		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			errCh <- fmt.Errorf("http serve %s: %w", addr, err)
+		}
+		close(errCh)
+	}()
+
+	select {
+	case err := <-errCh:
+		return err
+	case <-ctx.Done():
+		shutdownCtx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+		defer cancel()
+		if err := srv.Shutdown(shutdownCtx); err != nil {
+			return fmt.Errorf("http shutdown %s: %w", addr, err)
+		}
+		return nil
+	}
+}