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/app/webapp_test.go

@@ -0,0 +1,20 @@
+package app
+
+import (
+	"testing"
+
+	"github.com/dhao2001/mygo/internal/config"
+)
+
+func TestNewWebApp(t *testing.T) {
+	cfg := &config.Config{}
+
+	webApp := NewWebApp(cfg)
+
+	if webApp.Config != cfg {
+		t.Fatal("Config was not assigned")
+	}
+	if webApp.Version != AppVersion {
+		t.Errorf("Version = %q, want %q", webApp.Version, AppVersion)
+	}
+}