chore(webapp): add temporary /build-version compat shim

Serve the current build id at /build-version so an already-deployed client build
that polls it after a /build asset 404 recovers in a single reload instead of a
manual-reload dead-end. Endpoint only — deliberately no X-Build-Id response
header. Temporary; safe to remove once older clients have churned out.

Author: nicktrn

apps/webapp/server.ts

@@ -134,6 +134,15 @@ if (ENABLE_CLUSTER && cluster.isPrimary) {
   const port = process.env.REMIX_APP_PORT || process.env.PORT || 3000;
 
   if (process.env.HTTP_SERVER_DISABLED !== "true") {
+    // Back-compat shim: a previously-deployed client build polls this endpoint after a
+    // /build asset 404 and reloads once it reports a newer build id, letting those older
+    // tabs recover in a single reload. Temporary — safe to remove once older clients have
+    // churned out. Deliberately does NOT set an X-Build-Id response header.
+    app.get("/build-version", (_req, res) => {
+      res.set("Cache-Control", "no-store");
+      res.json({ version: build.assets.version });
+    });
+
     const socketIo: { io: IoServer } | undefined = build.entry.module.socketIo;
     const wss: WebSocketServer | undefined = build.entry.module.wss;
     const apiRateLimiter: RateLimitMiddleware = build.entry.module.apiRateLimiter;