Add full statistics page replacing placeholder: 4 KPI cards (total, Bestätigung,
Alternative, Uncodiert), stacked bar chart for gutachten types per KW, donut chart
for type distribution, grouped bar chart for therapy changes per KW, and horizontal
bar chart for therapy change reasons. Includes new backend endpoint and service
function combining sheet3/sheet4 data with KPI aggregation. Also adds feature
roadmap todo.md.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
