feat: ProductMember — team management for product backlogs

- Add ProductMember model (many-to-many User ↔ Product)
- Add productAccessFilter helper (owner OR member OR clause)
- Replace all ownership checks across actions and API routes
- Add addProductMemberAction / removeProductMemberAction / leaveProductAction
- Add TeamManager component in product settings (owner adds/removes Developers)
- Add LeaveProductButton in user settings (member leaves a product team)
- Regenerate Prisma Client after schema migration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/products/[id]/next-story/route.ts

@@ -1,5 +1,6 @@
 import { authenticateApiRequest } from '@/lib/api-auth'
 import { prisma } from '@/lib/prisma'
+import { productAccessFilter } from '@/lib/product-access'
 
 export async function GET(
   request: Request,
@@ -13,7 +14,7 @@ export async function GET(
   const { id } = await params
 
   const sprint = await prisma.sprint.findFirst({
-    where: { product_id: id, status: 'ACTIVE', product: { user_id: auth.userId } },
+    where: { product_id: id, status: 'ACTIVE', product: productAccessFilter(auth.userId) },
   })
   if (!sprint) {
     return Response.json({ error: 'Geen actieve Sprint gevonden' }, { status: 404 })