feat(migration): enforce ownerId requirement in Product table

- Removed all products without an owner to maintain data integrity.
- Updated ownerId column to be non-nullable.
- Modified foreign key constraint for ownerId to use ON DELETE CASCADE.

backend/prisma/schema.prisma

@@ -48,8 +48,8 @@ model Product {
   receiptAliases    ReceiptAlias[]
   tags              ProductTag[]
   nutrition         Nutrition?
-  ownerId           Int?
-  owner             User?           @relation(fields: [ownerId], references: [id], onDelete: SetNull)
+  ownerId           Int
+  owner             User            @relation(fields: [ownerId], references: [id], onDelete: Cascade)
   userProducts      UserProduct[]
   categoryId        Int?
   categoryRef       Category?       @relation(fields: [categoryId], references: [id], onDelete: SetNull)