feat: enhance purchase intentions with chat integration and offer tracking

- Added MongoDB chat/message integration for seller responses
- Improved seller response validation and default messaging
- Integrated automatic chat creation/update when responding to intentions
- Added highest custom offer fields to listing queries and models
- Enhanced listing data with top offer information for buyers

prisma/schema.prisma

@@ -346,6 +346,9 @@ model PurchaseIntention {
   buyer_id                    Int
   payment_method              String    @db.VarChar(20)
   payment_method_change_count Int       @default(0)
+  custom_offer_amount         Decimal?  @db.Decimal(12, 2)
+  custom_offer_currency       String?   @db.VarChar(20)
+  custom_offer_created_at     DateTime?
   status                      String    @default("pending") @db.VarChar(20)
   buyer_message               String?   @db.Text
   seller_response             String?   @db.Text