simplify and automate offer creation

api/prisma/schema.prisma

@@ -81,6 +81,7 @@ model Offer {
     want            Want       @relation(fields: [wantId], references: [id], onDelete: Cascade, onUpdate: Cascade)
 
     // only one offer per want for a user
-    @@unique([userId, wantId])
+    // @@unique([userId, wantId])
+    @@unique([listingId])
     @@map("offers")
 }