feat: add yt-dlp media helper

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/media/ytdlp.test.ts

@@ -0,0 +1,67 @@
+import { EventEmitter } from "node:events";
+import { PassThrough } from "node:stream";
+import { describe, expect, it, vi } from "vitest";
+import { createYtDlp } from "../../src/media/ytdlp";
+
+class FakeProcess extends EventEmitter {
+  stdout = new PassThrough();
+  stderr = new PassThrough();
+}
+
+describe("createYtDlp", () => {
+  it("reads YouTube metadata as JSON", async () => {
+    const proc = new FakeProcess();
+    const spawn = vi.fn(() => proc);
+    const ytdlp = createYtDlp({ spawn });
+
+    const result = ytdlp.getMetadata("https://youtu.be/video");
+    proc.stdout.write(
+      JSON.stringify({
+        title: "Song Title",
+        webpage_url: "https://youtube.com/watch?v=video",
+      }),
+    );
+    proc.stdout.end();
+    proc.emit("close", 0);
+
+    await expect(result).resolves.toEqual({
+      title: "Song Title",
+      webpageUrl: "https://youtube.com/watch?v=video",
+    });
+    expect(spawn).toHaveBeenCalledWith(
+      "yt-dlp",
+      [
+        "https://youtu.be/video",
+        "--dump-single-json",
+        "--no-playlist",
+        "--no-warnings",
+        "--quiet",
+      ],
+      { stdio: ["ignore", "pipe", "pipe"] },
+    );
+  });
+
+  it("reads direct audio URL", async () => {
+    const proc = new FakeProcess();
+    const ytdlp = createYtDlp({ spawn: vi.fn(() => proc) });
+
+    const result = ytdlp.getDirectAudioUrl("https://youtu.be/video");
+    proc.stdout.write("https://audio.example.com/stream\n");
+    proc.stdout.end();
+    proc.emit("close", 0);
+
+    await expect(result).resolves.toBe("https://audio.example.com/stream");
+  });
+
+  it("rejects when yt-dlp exits non-zero", async () => {
+    const proc = new FakeProcess();
+    const ytdlp = createYtDlp({ spawn: vi.fn(() => proc) });
+
+    const result = ytdlp.getMetadata("https://youtu.be/video");
+    proc.stderr.write("failed");
+    proc.stderr.end();
+    proc.emit("close", 1);
+
+    await expect(result).rejects.toThrow("yt-dlp failed with code 1");
+  });
+});