Reject task=translate on the server for models that cannot translate (D-044)

The UI already hid it; a direct API call to POST /api/transcribe or the live
WebSocket could still get the source language back labelled as a translation.

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

tests/transcribe.test.ts

@@ -6,6 +6,7 @@ import {
   createSidecar,
   createTranscribeHandlers,
   parseLanguageAndTask,
+  translateNotSupported,
   type Sidecar,
   type SidecarSegment,
   type SidecarTranscribeRequest,
@@ -262,6 +263,41 @@ describe("POST /api/transcribe", () => {
     }
   });
 
+  test("400 for translate on a model not trained for it; translate still works where it is (D-044)", async () => {
+    const sidecar = fakeSidecar();
+    const handlers = createTranscribeHandlers({
+      sidecar,
+      getSessionUser: sessionFromCookie,
+      tempDir: await makeTempDir(),
+    });
+    for (const model of [
+      "onnx-community/whisper-large-v3-turbo",
+      "onnx-community/whisper-small.en",
+    ]) {
+      const response = await handlers.post(
+        postRequest(`?model=${model}&task=translate`, pcmBody([0.1]), { cookie: "sid=ok" }),
+      );
+      expect(response.status).toBe(400);
+      expect(await response.text()).toContain("cannot translate");
+    }
+    expect(sidecar.requests).toHaveLength(0);
+
+    const ok = await handlers.post(
+      postRequest("?model=onnx-community/whisper-small&task=translate", pcmBody([0.1]), {
+        cookie: "sid=ok",
+      }),
+    );
+    expect(ok.status).toBe(200);
+  });
+
+  test("translateNotSupported: the shared check for the HTTP and live routes", () => {
+    expect(translateNotSupported("onnx-community/whisper-large-v3-turbo", "translate")).toContain(
+      "cannot translate",
+    );
+    expect(translateNotSupported("onnx-community/whisper-large-v3-turbo", "transcribe")).toBeUndefined();
+    expect(translateNotSupported("onnx-community/whisper-small", "translate")).toBeUndefined();
+  });
+
   test("503 with the reason when the sidecar is unavailable", async () => {
     const sidecar = fakeSidecar({ available: false, reason: "faster-whisper is not installed" });
     const handlers = createTranscribeHandlers({