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>

__tests__/lib/insights/burndown.test.ts

@@ -0,0 +1,57 @@
+import { describe, it, expect, vi } from 'vitest'
+
+vi.mock('@/lib/prisma', () => ({ prisma: {} }))
+
+import { computeBurndownDays } from '@/lib/insights/burndown'
+
+describe('computeBurndownDays', () => {
+  it('5-day sprint: remaining and ideal match spec', () => {
+    const start = new Date('2024-01-01T00:00:00.000Z')
+    const end = new Date('2024-01-05T00:00:00.000Z')
+
+    const tasks = [
+      { status: 'DONE', updated_at: new Date('2024-01-02T12:00:00.000Z') },
+      { status: 'DONE', updated_at: new Date('2024-01-04T12:00:00.000Z') },
+      { status: 'IN_PROGRESS', updated_at: new Date('2024-01-05T12:00:00.000Z') },
+    ]
+
+    const days = computeBurndownDays(tasks, start, end)
+
+    expect(days).toHaveLength(5)
+    expect(days.map(d => d.remaining)).toEqual([3, 2, 2, 1, 1])
+    expect(days.map(d => d.ideal)).toEqual([3, 2.25, 1.5, 0.75, 0])
+    expect(days.map(d => d.day)).toEqual([
+      '2024-01-01',
+      '2024-01-02',
+      '2024-01-03',
+      '2024-01-04',
+      '2024-01-05',
+    ])
+  })
+
+  it('returns empty array when end is before start', () => {
+    const start = new Date('2024-01-05T00:00:00.000Z')
+    const end = new Date('2024-01-01T00:00:00.000Z')
+    expect(computeBurndownDays([], start, end)).toEqual([])
+  })
+
+  it('single-day sprint has ideal = 0', () => {
+    const day = new Date('2024-01-01T00:00:00.000Z')
+    const tasks = [{ status: 'TO_DO', updated_at: new Date('2024-01-01T08:00:00.000Z') }]
+    const days = computeBurndownDays(tasks, day, day)
+    expect(days).toHaveLength(1)
+    expect(days[0].ideal).toBe(0)
+    expect(days[0].remaining).toBe(1)
+  })
+
+  it('all tasks done on first day: remaining drops to 0', () => {
+    const start = new Date('2024-01-01T00:00:00.000Z')
+    const end = new Date('2024-01-03T00:00:00.000Z')
+    const tasks = [
+      { status: 'DONE', updated_at: new Date('2024-01-01T10:00:00.000Z') },
+      { status: 'DONE', updated_at: new Date('2024-01-01T11:00:00.000Z') },
+    ]
+    const days = computeBurndownDays(tasks, start, end)
+    expect(days.map(d => d.remaining)).toEqual([0, 0, 0])
+  })
+})