feat(PBI-63): meerdere sprints per product + EXCLUDED + sprint-switcher (#161)

- Sprint lifecycle: ACTIVE→OPEN, COMPLETED→CLOSED, +ARCHIVED (FAILED behouden)
- TaskStatus: +EXCLUDED (overgeslagen door agent-loop via bestaande TO_DO filter)
- Cookie-gebaseerde actieve sprint per product (lib/active-sprint.ts)
- Route splitsen: /products/[id]/sprint/[sprintId] + /sprint redirect-page
- NavBar: gestapelde product/sprint dropdowns + BUILDING-badge derivatie
- Backlog selectie-modus + nieuwe-sprint-dialog (createSprintWithPbisAction)
- Migratie 20260507210000_sprint_lifecycle: ALTER TYPE RENAME (geen data-rewrite)
- Version bump 1.0.0 → 1.2.0

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

app/(app)/products/[id]/page.tsx

@@ -34,7 +34,7 @@ export default async function ProductBacklogPage({ params, searchParams }: Props
   if (!product) notFound()
 
   const [activeSprint, user] = await Promise.all([
-    prisma.sprint.findFirst({ where: { product_id: id, status: 'ACTIVE' } }),
+    prisma.sprint.findFirst({ where: { product_id: id, status: 'OPEN' } }),
     prisma.user.findUnique({ where: { id: session.userId! }, select: { active_product_id: true } }),
   ])