Enhance WHMCS integration with mTLS support and product mapping features. Added mTLS configuration options, updated API endpoints for mTLS status and fingerprint registration, and implemented product validation API. Updated database schema and documentation accordingly.

packages/database/prisma/schema.prisma

@@ -735,6 +735,8 @@ model WhmcsInstallation {
   integrationId String   @unique
   name          String
   apiSecret     String
+  mtlsClientFingerprint String?
+  mtlsClientSubject     String?
   isActive      Boolean  @default(true)
   metadata      Json?
   createdAt     DateTime @default(now()) @db.Timestamptz(3)