feat(insights): add getVelocity helper — DONE-tasks per completed sprint (#47)

Aggregates task.status=DONE counts across last N completed sprints
(default 5), filtered by productAccessFilter and returned in
chronological order for x-axis rendering.

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

lib/insights/velocity.ts

@@ -0,0 +1,57 @@
+import { prisma } from '@/lib/prisma'
+import { productAccessFilter } from '@/lib/product-access'
+
+export interface VelocitySprint {
+  sprintId: string
+  sprintGoal: string
+  productId: string
+  productName: string
+  doneCount: number
+  completedAt: string
+}
+
+export interface VelocityData {
+  sprints: VelocitySprint[]
+  productNames: { id: string; name: string }[]
+}
+
+export async function getVelocity(userId: string, sprintsBack = 5): Promise<VelocityData> {
+  const sprints = await prisma.sprint.findMany({
+    where: {
+      status: 'COMPLETED',
+      product: productAccessFilter(userId),
+    },
+    orderBy: { completed_at: 'desc' },
+    take: sprintsBack,
+    select: {
+      id: true,
+      sprint_goal: true,
+      completed_at: true,
+      product: { select: { id: true, name: true } },
+      tasks: { select: { status: true } },
+    },
+  })
+
+  // Reverse to chronological order (oldest first, for x-axis)
+  const chronological = [...sprints].reverse()
+
+  const result: VelocitySprint[] = chronological.map(sprint => ({
+    sprintId: sprint.id,
+    sprintGoal: sprint.sprint_goal,
+    productId: sprint.product.id,
+    productName: sprint.product.name,
+    doneCount: sprint.tasks.filter(t => t.status === 'DONE').length,
+    completedAt: sprint.completed_at!.toISOString(),
+  }))
+
+  const seen = new Set<string>()
+  const productNames: { id: string; name: string }[] = []
+  for (const s of result) {
+    if (!seen.has(s.productId)) {
+      seen.add(s.productId)
+      productNames.push({ id: s.productId, name: s.productName })
+    }
+  }
+
+  return { sprints: result, productNames }
+}