fix: add @unique to Transaction.requestId for one-to-one relation

Prisma requires unique fields on the defining side of one-to-one relations.
Fixes P1012 validation error during prisma generate.

backend/prisma/schema.prisma

@@ -51,7 +51,7 @@ model Transaction {
   watchEvent   WatchEvent? @relation(fields: [watchEventId], references: [id])
   
   // For spent transactions
-  requestId   String? @map("request_id")
+  requestId   String? @unique @map("request_id")
   request     ContentRequest? @relation(fields: [requestId], references: [id])
   
   // Solana transaction reference