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_test.go

@@ -0,0 +1,48 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/dhao2001/mygo/internal/app"
+	"github.com/dhao2001/mygo/internal/config"
+)
+
+func TestVersionRoute(t *testing.T) {
+	webApp := app.NewWebApp(&config.Config{})
+	router := NewRouter(webApp)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/version", nil)
+	rec := httptest.NewRecorder()
+
+	router.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if got := rec.Header().Get("Content-Type"); got != "application/json; charset=utf-8" {
+		t.Fatalf("content-type = %q, want %q", got, "application/json; charset=utf-8")
+	}
+
+	var body struct {
+		Version string `json:"version"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("unmarshal response: %v", err)
+	}
+
+	if body.Version != app.AppVersion {
+		t.Errorf("version = %q, want %q", body.Version, app.AppVersion)
+	}
+}
+
+func TestAddress(t *testing.T) {
+	got := Address(config.ServerConfig{Host: "127.0.0.1", Port: 8080})
+	want := "127.0.0.1:8080"
+	if got != want {
+		t.Errorf("Address() = %q, want %q", got, want)
+	}
+}