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>

lib/product-access.ts

@@ -0,0 +1,18 @@
+import { prisma } from '@/lib/prisma'
+
+const accessFilter = (userId: string) => ({
+  OR: [
+    { user_id: userId },
+    { members: { some: { user_id: userId } } },
+  ],
+})
+
+export async function getAccessibleProduct(productId: string, userId: string) {
+  return prisma.product.findFirst({
+    where: { id: productId, ...accessFilter(userId) },
+  })
+}
+
+export function productAccessFilter(userId: string) {
+  return accessFilter(userId)
+}