feat: prefer derived mp4 playback with fallback

apps/web/src/__tests__/prefer-derived.test.ts

@@ -0,0 +1,18 @@
+import { test, expect } from "bun:test";
+import { pickVideoPlaybackVariant } from "../../app/lib/playback";
+
+test("prefer mp4 derived over original", () => {
+  const picked = pickVideoPlaybackVariant({
+    originalMimeType: "video/x-matroska",
+    variants: [{ kind: "video_mp4", size: 720, key: "derived/video/a.mp4" }],
+  });
+  expect(picked).toEqual({ kind: "video_mp4", size: 720 });
+});
+
+test("falls back to original when no derived", () => {
+  const picked = pickVideoPlaybackVariant({
+    originalMimeType: "video/x-matroska",
+    variants: [],
+  });
+  expect(picked).toEqual({ kind: "original" });
+});