Translate transcripts with Opus-MT in a dedicated translation worker

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/index.ts

@@ -223,7 +223,8 @@ async function productionServeOptions(): Promise<ServeOptions> {
  * Bun's bundler does not currently follow `new Worker(...)` graphs (verified
  * with Bun 1.3: both `bun build` and the dev server leave the call untouched),
  * so worker-like entrypoints are bundled at startup and served from stable
- * URLs. The UI spawns `new Worker("/inference.worker.js")`; the service
+ * URLs. The UI spawns `new Worker("/inference.worker.js")` (and
+ * "/translate.worker.js" for transcript translation, D-WS4b); the service
  * worker registers as "/sw.js".
  */
 async function bundleEntrypoint(relativePath: string): Promise<string> {
@@ -249,6 +250,8 @@ async function developmentServeOptions(): Promise<ServeOptions> {
   const { default: index } = await import("./index.html");
 
   const workerScript = await bundleEntrypoint("./worker/inference.worker.ts");
+  // Transcript translation runs in its own worker (D-WS4b).
+  const translateWorkerScript = await bundleEntrypoint("./worker/translate.worker.ts");
   // Dev SW: empty precache list — assets are unhashed here, so the shell is
   // runtime-cached network-first instead (see src/pwa/sw.ts).
   const swScript = injectPrecacheManifest(
@@ -283,6 +286,11 @@ async function developmentServeOptions(): Promise<ServeOptions> {
       if (pathname.endsWith("/inference.worker.js")) {
         return withIsolationHeaders(new Response(workerScript, { headers: scriptHeaders }));
       }
+      if (pathname.endsWith("/translate.worker.js")) {
+        return withIsolationHeaders(
+          new Response(translateWorkerScript, { headers: scriptHeaders }),
+        );
+      }
       if (pathname.endsWith("/sw.js")) {
         return withIsolationHeaders(
           new Response(swScript, { headers: { ...scriptHeaders, "Cache-Control": "no-cache" } }),