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.go

@@ -0,0 +1,19 @@
+package app
+
+import (
+	"github.com/dhao2001/mygo/internal/config"
+)
+
+// WebApp contains application-wide runtime dependencies and metadata.
+type WebApp struct {
+	Config  *config.Config
+	Version string
+}
+
+// NewWebApp creates the application dependency container for the HTTP server.
+func NewWebApp(cfg *config.Config) *WebApp {
+	return &WebApp{
+		Config:  cfg,
+		Version: AppVersion,
+	}
+}