OpenAPI documentation fixes

[FliegendeWurst]

1. Revive the generate-openapi script by putting it in top-level scripts.
2. Fix some types in the documentation
3. Docs/Attachments: mention the other possible type "file"
4. Add OpenAPI for login route

For step 1, I could also put the script in server/scripts if you prefer.

[FliegendeWurst]

CI failure unrelated, I think

[eliandoran]

Thanks!