test: stub server-only under Vitest

The scenario-metrics route test now imports a module chain
(route → overlay.ts) that carries `import "server-only"`, which Vite can't
resolve to a Node entry. Alias it to an empty stub so the guard stays in app
code while Node-env tests load. Full suite: 600 passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

vitest.config.ts

@@ -15,6 +15,9 @@ export default defineConfig({
   resolve: {
     alias: {
       "@": path.resolve(__dirname, "./src"),
+      // `server-only` is a marker package with no Node-resolvable entry under
+      // Vite; stub it so tests that import a server module chain still load.
+      "server-only": path.resolve(__dirname, "./vitest/server-only-stub.ts"),
     },
   },
 });

vitest/server-only-stub.ts

@@ -0,0 +1,6 @@
+// Empty stub for the `server-only` marker package under Vitest. The real
+// package resolves to an empty module on the server and a throwing one in the
+// browser via export conditions Vite doesn't apply here; aliasing it keeps
+// server-only guards in place in app code without breaking Node-env tests that
+// import a module chain containing one. See vitest.config.ts.
+export {};