Allow in progress recording downloads

[Sayan-]

Description

We also want to be able to show recordings of the session so far. The way we've hooked up ffmpeg allows this already! The work in this pr is to:

1. Relax constraints around downloading in progress recordings
2. Instead, we'll check the file size and return a 202 if the recording doesn't have a reasonable amount of data. For the moment we've picked 100 bytes, which is substantially smaller than what ffmpeg will flush to disk

Also lurked in some minor config validation + cleaned up the Start logic as recommended by my cursor agents

Testing

Via the local docker set up:

1. Build and run the container with the recording server
2. Start recording
3. Immediately hit the download -> confirm a 202 response
4. (wait for a bit)
5. Hit the download again -> confirm valid video output
6. Stop recording
7. Download -> confirm valid video output