Document scanner's v5 workflow

[zoriya]

Part of #597

For v5, we will rework a lot of things in the scanning process. This PR creates a document that details the scanning workflow.

Big changes compared to current version include:

• The api knows & register even episodes we don't have videos for (or that are not aired).
  • Episodes missing a video will be displayed in the app accordingly.
  • This allows the user to know there's an episode missing and not inadvertently skip one.
  • We could use this information in admin dashboards.
  • Fetching all episodes was already needed in some cases, this would make them easier.
• Videos are saved in database before we register their series/movie.
  • We can display them in the interface as loading or fetching-data.
  • If metadata matching fails, we can still play them.
  • We can manually correct the guess & continue the registration flow.
• Metadata guesses are saved in database (just guesses based on file name, not external db data)
  • This allows correction to also apply to other episodes of the same series.
  • It gives better information to the user as to why it was guessed this way (or at least it improves the debugging experience for us).