New GET /cases/export endpoint generates .xlsx with openpyxl using the
same filters as the case list (year, fallgruppe, ICD status, search).
Role-aware columns: admins see patient names, DAK staff does not.
Frontend adds a Download button next to the filter bar with loading
state. Refactors shared query logic into _build_case_query helper.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
