Import a reference workbook as a model plan

Parse an uploaded .xlsx into a draft model and return it for review. Nothing is persisted — the importer cannot know which ledgers feed the workbook's data rows, and guessing would produce a model that renders a clean grid of wrong numbers. The response carries a confidence score, the sheets skipped as raw data exports, and the formulas that could not be translated, so the caller can resolve them before calling /models/build.