Make Classification.confidence non-nullable

The Classification.confidence field is currently a nullable float, but we have not needed null confidences for a long time. CreateClassifications requires a confidence, and CreateClassification requires it only when using worker runs, and defaults to 1 for manual classifications, so the APIs always require a confidence. There are exactly 0 classifications on all Teklia-managed instances without a confidence.

Please remove null=True on the field and update the SQLite export structure so that the relevant column is marked as NOT NULL. Due to null confidences being impossible enough, bumping the export version is overkill here. Make sure that the confidence is now reported as non-nullable in the API documentation.