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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>