BillingTopupService кредитует tenants.balance_rub (bcmath) и пишет
append-only строку balance_transactions(type='topup'). BillingController
+ route POST /api/billing/topup под [auth:sanctum, tenant]. MVP-stub:
без платёжного шлюза (ЮKassa — post-Б-1).
Sprint 2 Plan C, audit E1 (backend).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Дмитрий