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>

__tests__/api/security.test.ts

@@ -197,7 +197,7 @@ describe('GET /api/products/:id/next-story', () => {
       expect.objectContaining({
         where: expect.objectContaining({
           product_id: 'prod-other',
-          status: 'ACTIVE',
+          status: 'OPEN',
           product: expect.objectContaining({
             OR: expect.arrayContaining([{ user_id: 'user-1' }]),
           }),