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/router.go

@@ -0,0 +1,32 @@
+package server
+
+import (
+	"github.com/gin-gonic/gin"
+
+	"github.com/dhao2001/mygo/internal/app"
+	"github.com/dhao2001/mygo/internal/handler"
+)
+
+// NewRouter builds the Gin router and registers API routes.
+func NewRouter(webApp *app.WebApp) *gin.Engine {
+	router := gin.Default()
+
+	rootGroup := router.Group("/api/v1")
+
+	publicGroup := rootGroup.Group("")
+	setupVersionRoutes(publicGroup, handler.NewVersionHandler(webApp.Version))
+
+	authGroup := rootGroup.Group("")
+	setupProtectedRoutes(authGroup)
+
+	return router
+}
+
+func setupVersionRoutes(rg *gin.RouterGroup, h *handler.VersionHandler) {
+	rg.GET("/version", h.Get)
+}
+
+func setupProtectedRoutes(rg *gin.RouterGroup) {
+	// Protected routes will be registered after auth middleware exists.
+	_ = rg
+}