Each variant has a unique SKU or identifier

Duplicate SKUs break every system that uses SKU as a lookup key: warehouse management, shipping label generation, return processing, and analytics attribution. CWE-694 (Use of Multiple Resources with Duplicate Identifier) applies directly — two variants sharing SHIRT-RED-M means a barcode scan on the warehouse floor resolves ambiguously, order fulfillment picks the wrong record, and return lookups return both variants. Without a database-level UNIQUE constraint, application-level deduplication checks are vulnerable to race conditions under concurrent insertion — two simultaneous product imports can both pass validation and both write the same SKU.

Add a @unique constraint to the sku field in prisma/schema.prisma so the database rejects duplicate SKUs at the storage layer.

model ProductVariant {
  id        String  @id @default(cuid())
  sku       String  @unique
  productId String
  product   Product @relation(fields: [productId], references: [id])
  size      String
  color     String
}

In src/app/api/products/route.ts, catch the Prisma unique constraint error (P2002) and return a 409 with a clear message rather than letting it surface as a 500.