Remove dedupeHash and duplicate skipping from CSV upload

Drop dedupeHash field and unique constraint from Transaction model.
Remove skipDuplicates from createMany. All rows in every upload are
now inserted unconditionally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -63,11 +63,9 @@ model Transaction {
   type        TransactionType
   category    String?
   notes       String?
-  dedupeHash  String
   createdAt   DateTime        @default(now())
   updatedAt   DateTime        @updatedAt
 
-  @@unique([dedupeHash])
   @@index([accountId, date])
   @@index([date])
   @@index([budgetId])