Import Categorisation Refactor

Server-Side Rule Creation

Rule creation during bank import is now handled entirely by the server. Previously, the import preview performed keyword extraction and rule creation in the browser — this has been moved to a dedicated API endpoint.

• Single source of truth — normalisation, keyword extraction, and rule creation logic now lives exclusively in the API. No duplicated business logic in the frontend.
• Smarter similar-row detection — when you categorise a transaction, the server identifies other rows in the batch with matching merchants and applies the same account automatically.
• Correct upsert behaviour — changing a categorisation during import updates the existing rule rather than silently failing.

This is an internal refactor with no change to user-facing behaviour. The import preview works exactly as before — categorise one transaction and similar ones update automatically.