feat: getBurndownData helper + computeBurndownDays (lib/insights/burndown.ts) (#34)

Server-side aggregatie per active sprint: bouwt time-series met remaining en ideal per dag.
Inclusief 4 Vitest-unit-tests voor de pure computeBurndownDays functie.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/insights/burndown.ts

@@ -0,0 +1,87 @@
+import { prisma } from '@/lib/prisma'
+import { productAccessFilter } from '@/lib/product-access'
+
+export interface BurndownDay {
+  day: string
+  remaining: number
+  ideal: number
+}
+
+export interface BurndownSprint {
+  sprintId: string
+  productId: string
+  productName: string
+  sprintGoal: string
+  days: BurndownDay[]
+}
+
+const DAY_MS = 86_400_000
+
+function toUTCMidnight(d: Date): Date {
+  return new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate()))
+}
+
+export function computeBurndownDays(
+  tasks: { status: string; updated_at: Date }[],
+  startDate: Date,
+  endDate: Date,
+): BurndownDay[] {
+  const start = toUTCMidnight(startDate)
+  const end = toUTCMidnight(endDate)
+  const total = tasks.length
+  // n = number of intervals (end - start in days)
+  const n = Math.round((end.getTime() - start.getTime()) / DAY_MS)
+
+  const days: BurndownDay[] = []
+
+  for (let i = 0; ; i++) {
+    const dayStart = new Date(start.getTime() + i * DAY_MS)
+    if (dayStart > end) break
+
+    const nextDay = new Date(dayStart.getTime() + DAY_MS)
+    const done = tasks.filter(t => t.status === 'DONE' && t.updated_at < nextDay).length
+    const ideal = n === 0 ? 0 : Math.round((total * (n - i) / n) * 100) / 100
+
+    days.push({
+      day: dayStart.toISOString().slice(0, 10),
+      remaining: total - done,
+      ideal,
+    })
+  }
+
+  return days
+}
+
+export async function getBurndownData(userId: string): Promise<BurndownSprint[]> {
+  const now = new Date()
+
+  const sprints = await prisma.sprint.findMany({
+    where: {
+      status: 'ACTIVE',
+      product: productAccessFilter(userId),
+    },
+    select: {
+      id: true,
+      sprint_goal: true,
+      created_at: true,
+      completed_at: true,
+      product: { select: { id: true, name: true } },
+      tasks: { select: { status: true, updated_at: true } },
+    },
+  })
+
+  return sprints
+    .map(sprint => {
+      const endDate = sprint.completed_at ?? now
+      if (endDate <= sprint.created_at) return null
+
+      return {
+        sprintId: sprint.id,
+        productId: sprint.product.id,
+        productName: sprint.product.name,
+        sprintGoal: sprint.sprint_goal,
+        days: computeBurndownDays(sprint.tasks, sprint.created_at, endDate),
+      }
+    })
+    .filter((s): s is BurndownSprint => s !== null)
+}