[Idea] CSV forecast import

# Rationale

* Allow importing forecasts made in the "Personal Prediction Log" Android app
* Allow people to collect forecasts in an Excel sheet and export as CSV
* It's possible to write CSV completely by hand

# Notes

The format of different CSVs may differ very much e.g. some might be missing required fields. What to do then? The order of the fields will also be different, so it must be configurable somehow.

Should the API support CSV import or should it all be done in the cleoc client or the web frontend?

No files, tests, or entry points are identified. Start by reviewing the API, cleoc client, and web frontend boundaries, then clarify the CSV mapping, missing-field behavior, and where import should live; done requires an agreed implementation scope.

Written by the indexing model from the issue text.