Devlog #2: Video upload flow with TikTok API

Uploading a video via the TikTok Content Posting API is not just a POST request with a file. It's a multi-step process.

First we initialize the upload: we send video metadata (size, duration) and receive back an upload URL and upload ID. Then we transfer the file in chunks to that URL. After the transfer completes, we notify the API that the upload is done.

Chunked upload matters: video files can be large, and transferring them in a single request is unreliable. If the connection is interrupted midway, you can resume from the correct chunk.

This is the first place where we got an explicit intermediate state — "file is uploading."