Remove unused model constants

- Company: COMPANY_LEVEL, CUSTOMER_LEVEL (never referenced)
- Payment: all 5 PAYMENT_MODE_* constants (never referenced)
- Transaction: PENDING (never referenced)

RecurringInvoice constants (ACTIVE, ON_HOLD, NONE, COUNT, DATE) are kept
as they are used via hardcoded strings in services, factories, and migrations.

app/Models/Transaction.php

@@ -20,8 +20,6 @@ class Transaction extends Model
         'transaction_date',
     ];
 
-    public const PENDING = 'PENDING';
-
     public const FAILED = 'FAILED';
 
     public const SUCCESS = 'SUCCESS';