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.

• ID: ecommerce-catalog.variant-pricing.unique-sku

• Severity: high

• What to look for: Count all variant records in seed data, fixtures, or sample responses. For each variant, check whether a SKU field exists. Verify the SKU has a database-level unique constraint (@unique in Prisma, UNIQUE in SQL). Enumerate any duplicate SKUs found in the data.

• Pass criteria: All variants have a SKU field with a database-level unique constraint, and 0 duplicate SKUs exist in seed data or fixtures. Report: "X variants checked, 0 duplicates found, unique constraint: [present/absent]."

• Fail criteria: No SKU field exists on the variant model, or the SKU field lacks a unique constraint, or at least 1 duplicate SKU exists in seed data.

• Skip (N/A) when: The project has no variants (all products have a single SKU) — confirmed by absence of a variant model or relation in the schema.

• Cross-reference: For SKU generation and uniqueness patterns, the Database Design & Operations audit covers unique constraint design.

• Cross-reference: For SKU display in order management, the Pre-Launch audit covers data integrity in transactional flows.

• Cross-reference: For inventory lookup by SKU, the check ecommerce-catalog.inventory.stock-initialized in this audit covers stock tracking per variant.

• Detail on fail: "Variant schema in prisma/schema.prisma has no SKU field" or "2 variants share the same SKU: 'SHIRT-RED-M' in prisma/seed.ts"

• Remediation: Add a unique SKU field to the variant schema in prisma/schema.prisma:

  model ProductVariant {
    id String @id @default(cuid())
    sku String @unique
    productId String
    size String
    color String
  }