feat(02-05): Payment model with FIFO allocation and void
- Add PaymentMethod (CASH, BANK_TRANSFER) and PaymentStatus (COMPLETED, VOIDED) enums
- Add Payment model with idempotency key, journal entry link, void fields
- Add PaymentAllocation model for FIFO invoice allocation tracking
- Add Payment/PaymentAllocation relations to Subscriber, Invoice, User
- Update TENANT_SCOPED_MODELS with "payment" and "paymentAllocation"
- Add payment/paymentAllocation query extensions in withTenantContext()
- Implement recordPayment() with FIFO allocation, overpayment credit balance
- Implement voidPayment() with reversing journal entries
- Implement getSubscriberPaymentHistory() with pagination
- Run migration: 20260304154606_add_payment_model
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