feat: prefer derived mp4 playback with fallback

apps/web/app/lib/playback.ts

@@ -0,0 +1,22 @@
+type Variant = {
+  kind: "video_mp4";
+  size: number;
+  key: string;
+};
+
+type PlaybackInput = {
+  originalMimeType: string | null | undefined;
+  variants: Variant[];
+};
+
+export function pickVideoPlaybackVariant(input: PlaybackInput):
+  | { kind: "video_mp4"; size: number }
+  | { kind: "original" } {
+  const mp4Variant = input.variants.find(
+    (variant) => variant.kind === "video_mp4" && variant.size === 720,
+  );
+  if (mp4Variant) {
+    return { kind: "video_mp4", size: mp4Variant.size };
+  }
+  return { kind: "original" };
+}