Fix offer retrieval method and add GDPR acknowledgment

Return the result from `getOffersById` method to fix route handling. Add `id` field in offers selection query and introduce `gdpr_acknowledgement` to the user model and mock data for GDPR compliance.

prisma/schema.prisma

@@ -86,6 +86,8 @@ model User {
   created_at       DateTime @default(now())
   updated_at       DateTime @default(now()) @updatedAt
 
+  gdpr_acknowledgement DateTime @default(now())
+
   Role          Role            @relation(fields: [roleId], references: [id])
   UserHasCrypto UserHasCrypto[]
   TradeGiven    Trade[]         @relation("Giver")