feat(server): embed SPA dist with fallback handler; single-binary deploy

internal/transport/http/router.go

@@ -17,6 +17,9 @@ type RouterDeps struct {
 	HealthCheck func(*http.Request) error
 	// ReadyCheck is invoked from GET /readyz with the same contract.
 	ReadyCheck func(*http.Request) error
+	// SPAHandler 为 nil 时禁用前端 fallback；非 nil 时挂到 chi NotFound，
+	// 把未匹配的请求交给 SPA handler 处理（静态文件 + index.html 回退）。
+	SPAHandler http.Handler
 }
 
 // NewRouter builds the application's chi.Router with the standard
@@ -46,5 +49,9 @@ func NewRouter(deps RouterDeps) chi.Router {
 		_, _ = w.Write([]byte("ok"))
 	})
 
+	if deps.SPAHandler != nil {
+		r.NotFound(deps.SPAHandler.ServeHTTP)
+	}
+
 	return r
 }