fix(prisma): add unique constraint to Chat model in Postgres

Generated migration to add unique index on instanceId and remoteJid.

prisma/postgresql-migrations/20251122003044_add_chat_instance_remotejid_unique/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+CREATE UNIQUE INDEX "Chat_instanceId_remoteJid_key" ON "Chat"("instanceId", "remoteJid");

prisma/postgresql-schema.prisma

@@ -132,6 +132,7 @@ model Chat {
   instanceId     String
   unreadMessages Int       @default(0)
 
+  @@unique([instanceId, remoteJid])
   @@index([instanceId])
   @@index([remoteJid])
 }