feat: ST-601-ST-612 M6 polish, beveiliging en launch-ready

- ST-601/602: loading skeletons en error boundary
- ST-603: Sonner toasts op alle CRUD-operaties
- ST-604: DemoTooltip op uitgeschakelde knoppen
- ST-605: KeyboardSensor dnd-kit, Escape sluit modals
- ST-606: min-width banner < 1024px
- ST-607: WCAG AA aria-labels en skip link
- ST-608: rate limiting login (10/min) en registratie (5/uur)
- ST-609: security integratietests cross-user toegang (7 tests)
- ST-610: GitHub Actions CI/CD workflow
- ST-611: README met quickstart, deployment en API-docs
- ST-612: Lars-flow acceptatiechecklist
- fix: settings toont gebruikersnaam i.p.v. interne id
- fix: seed idempotent, testdata altijd gekoppeld aan demo-gebruiker

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/sprint/sprint-backlog.tsx

@@ -4,10 +4,11 @@ import { useState, useTransition, useEffect } from 'react'
 import { useRouter } from 'next/navigation'
 import {
   DndContext, DragEndEvent, DragOverEvent, DragStartEvent, DragOverlay,
-  PointerSensor, useSensor, useSensors, closestCenter,
+  KeyboardSensor, PointerSensor, useSensor, useSensors, closestCenter,
 } from '@dnd-kit/core'
 import {
   SortableContext, useSortable, verticalListSortingStrategy, arrayMove,
+  sortableKeyboardCoordinates,
 } from '@dnd-kit/sortable'
 import { CSS } from '@dnd-kit/utilities'
 import { toast } from 'sonner'
@@ -68,6 +69,7 @@ function SortableSprintRow({
     >
       {!isDemo && (
         <span {...attributes} {...listeners} onClick={e => e.stopPropagation()}
+          aria-label="Versleep om te sorteren"
           className="text-muted-foreground cursor-grab active:cursor-grabbing shrink-0 select-none text-sm">
           ⠿
         </span>
@@ -113,7 +115,10 @@ export function SprintBacklogLeft({ sprintId, stories, isDemo, onSelectStory, se
   const order = sprintStoryOrder[sprintId] ?? stories.map(s => s.id)
   const orderedStories = order.map(id => storyMap[id]).filter(Boolean)
 
-  const sensors = useSensors(useSensor(PointerSensor, { activationConstraint: { distance: 5 } }))
+  const sensors = useSensors(
+    useSensor(PointerSensor, { activationConstraint: { distance: 5 } }),
+    useSensor(KeyboardSensor, { coordinateGetter: sortableKeyboardCoordinates })
+  )
 
   function handleDragEnd(event: DragEndEvent) {
     const { active, over } = event