feat(auth): referral agent code and auto-assign agent codes

Validate optional referrer code at OTP registration and persist assignedAgentId. Auto-generate unique W-XXXX codes when creating or promoting agents and backfill on agent login when missing.

Co-authored-by: Cursor <cursoragent@cursor.com>

prisma/migrations/20261004100000_otp_referrer_agent/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "OtpChallenge" ADD COLUMN "assignedAgentId" TEXT;

prisma/schema.prisma

@@ -290,15 +290,16 @@ model AdminUser {
 }
 
 model OtpChallenge {
-  id         String   @id @default(cuid())
-  phone      String
-  code       String
-  firstName  String?
-  lastName   String?
-  isRegister Boolean  @default(false)
-  attempts   Int      @default(0)
-  expiresAt  DateTime
-  createdAt  DateTime @default(now())
+  id              String   @id @default(cuid())
+  phone           String
+  code            String
+  firstName       String?
+  lastName        String?
+  isRegister      Boolean  @default(false)
+  assignedAgentId String?
+  attempts        Int      @default(0)
+  expiresAt       DateTime
+  createdAt       DateTime @default(now())
 
   @@index([phone])
 }