feat(billing): wallet/transactions/invoices read API (E3)

GET /api/billing/wallet (баланс + тариф + runway), /transactions
(пагинированный balance_transactions с фильтром type), /invoices
(saas_invoices, real-but-empty до Б-1). TariffPlan модель +
Tenant::tariff() relation + BalanceTransactionFactory.

Sprint 2 Plan C, audit E3 (backend).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/app/Models/BalanceTransaction.php

@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace App\Models;
 
+use Database\Factories\BalanceTransactionFactory;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
@@ -19,6 +21,9 @@ use Illuminate\Database\Eloquent\Relations\BelongsTo;
  */
 class BalanceTransaction extends Model
 {
+    /** @use HasFactory<BalanceTransactionFactory> */
+    use HasFactory;
+
     public const TYPE_TRIAL_BONUS = 'trial_bonus';
 
     public const TYPE_TOPUP = 'topup';