Implement web API foundation

Add application container, Gin router, graceful shutdown handler,
and version endpoint. This establishes the skeleton for the WebDisk
HTTP API as described in the architecture.

- Add internal/app/WebApp for runtime dependencies and version
- Add internal/server/router with GET /api/v1/version route
- Add graceful shutdown runner with signal handling in cmd/serve
- Add internal/api/ErrorResponse for standard HTTP error body
- Update roadmap, architecture, and decisions documentation

internal/server/server.go

@@ -0,0 +1,49 @@
+package server
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/dhao2001/mygo/internal/config"
+)
+
+// Address returns the HTTP listen address for a server config.
+func Address(cfg config.ServerConfig) string {
+	return fmt.Sprintf("%s:%d", cfg.Host, cfg.Port)
+}
+
+// RunWithGracefulShutdown starts an HTTP server and shuts it down when ctx is canceled.
+func RunWithGracefulShutdown(ctx context.Context, addr string, handler http.Handler) error {
+	if handler == nil {
+		return errors.New("handler: must not be nil")
+	}
+
+	httpServer := &http.Server{
+		Addr:              addr,
+		Handler:           handler,
+		ReadHeaderTimeout: 5 * time.Second,
+	}
+
+	errCh := make(chan error, 1)
+	go func() {
+		errCh <- httpServer.ListenAndServe()
+	}()
+
+	select {
+	case <-ctx.Done():
+		shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		if err := httpServer.Shutdown(shutdownCtx); err != nil {
+			return fmt.Errorf("shutdown server: %w", err)
+		}
+		return nil
+	case err := <-errCh:
+		if errors.Is(err, http.ErrServerClosed) {
+			return nil
+		}
+		return fmt.Errorf("listen and serve: %w", err)
+	}
+}