add flake devenv, dev docs, adapt utils.sh

[tennox]

This is what I needed to set up so I could start a server.

The server does not have any data though, and I did not manage to run update-database:

❯ ./utils.sh update-database
user: tennox
token: github_pat_11...
concurrency: 8
progress: 0/341
progress: 1/341
progress: 2/341
progress: 5/341
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progress: 4/341
GitHub: failed [nixos-17.03 [2024-03-01T00:00:00Z - 2024-03-31T00:00:00Z]]: Unknown: Status {statusCode = 401, statusMessage = "Unauthorized"} {"message":"Bad credentials","documentation_url":"https://docs.github.com/rest","status":"401"}
Failure: Nixos_17_03 [2024-03-01T00:00:00Z - 2024-03-31T00:00:00Z]
progress: 10/341