docs(ST-1109.10): document PbiStatus enum, sprint-close cascade, and filter UI

- docs/scrum4me-architecture.md: pbis-table updated with status column +
  index; PbiStatus enum + Pbi model in the Prisma schema sample;
  cascade-on-sprint-close rule documented inline
- docs/scrum4me-styling.md: short note pointing to PBI_STATUS_LABELS /
  PBI_STATUS_COLORS in components/shared/pbi-status-select.tsx so future
  components don't ad-hoc-copy the color map
- docs/plans/ST-1109-pbi-status.md: in-repo mirror of the approved plan
  (per feedback_plan_location memory) with cascade pseudo-code and
  end-to-end verification checklist

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/scrum4me-styling.md

@@ -280,6 +280,12 @@ import { Badge } from '@/components/ui/badge'
 <Badge variant="secondary">3 taken</Badge>
 ```
 
+**PBI-status (READY / BLOCKED / DONE):** hergebruikt bestaande tokens —
+`status-todo` voor READY, `status-blocked` voor BLOCKED, `status-done` voor
+DONE. Centraal gedefinieerd in `components/shared/pbi-status-select.tsx`
+(`PBI_STATUS_LABELS`, `PBI_STATUS_COLORS`); importeer die in plaats van
+kleuren ad-hoc te kopiëren.
+
 ### Dialog (bevestigingsdialogen)
 
 ```tsx