feat: add mp4 transcode job and variant record

apps/worker/src/__tests__/transcode-plan.test.ts

@@ -0,0 +1,10 @@
+import { test, expect } from "bun:test";
+import { shouldTranscodeToMp4 } from "../transcode";
+
+test("transcode runs for non-mp4 videos", () => {
+  expect(shouldTranscodeToMp4({ mimeType: "video/x-matroska" })).toBe(true);
+});
+
+test("transcode skips for mp4", () => {
+  expect(shouldTranscodeToMp4({ mimeType: "video/mp4" })).toBe(false);
+});