feat(ST-004): add lars as member of Scrum4Me product

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

prisma/seed.ts

@@ -114,6 +114,11 @@ async function main() {
 
   console.log(`Product created: ${product.name} (id: ${product.id}, owner: demo)`)
 
+  await prisma.productMember.create({
+    data: { product_id: product.id, user_id: user.id },
+  })
+  console.log(`  Added ${user.username} as member of ${product.name}`)
+
   const milestones = await loadBacklog(root)
   console.log(`Loaded backlog: ${milestones.length} milestones, ${milestones.reduce((acc, m) => acc + m.stories.length, 0)} stories`)