Batch historical balance writes (#356)

## Summary
- Batch calculated historical account balances with one upsert
- Keep existing balance dates untouched
- Add regression coverage for one account_balances write query

Fixes PHP-LARAVEL-1C

## Tests
- php artisan test --compact
tests/Feature/OpenBanking/BalanceSyncServiceTest.php
- vendor/bin/pint --dirty --format agent

app/Services/Banking/BalanceSyncService.php

@@ -4,8 +4,10 @@ namespace App\Services\Banking;
 
 use App\Contracts\BankingProviderInterface;
 use App\Models\Account;
+use App\Models\AccountBalance;
 use Carbon\Carbon;
 use Illuminate\Support\Facades\Log;
+use Illuminate\Support\Str;
 
 class BalanceSyncService
 {
@@ -87,18 +89,28 @@ class BalanceSyncService
 
         $runningBalance = $referenceBalance->balance;
         $referenceDate = $referenceBalance->balance_date->toDateString();
+        $now = now();
+        $rows = [];
 
         foreach ($dailyTotals as $date => $sum) {
             if ($date < $referenceDate && ! isset($existingDates[$date])) {
-                $account->balances()->create([
+                $rows[] = [
+                    'id' => (string) Str::uuid(),
+                    'account_id' => $account->id,
                     'balance_date' => $date,
                     'balance' => $runningBalance,
-                ]);
+                    'created_at' => $now,
+                    'updated_at' => $now,
+                ];
             }
 
             $runningBalance -= (int) $sum;
         }
 
+        if ($rows !== []) {
+            AccountBalance::upsert($rows, ['account_id', 'balance_date'], ['balance', 'updated_at']);
+        }
+
         Log::info('Calculated historical balances', [
             'account_id' => $account->id,
             'reference_date' => $referenceDate,

tests/Feature/OpenBanking/BalanceSyncServiceTest.php

@@ -59,6 +59,39 @@ test('calculateHistoricalBalances derives balances from transactions', function
     expect($feb5->balance)->toBe(85000);
 });
 
+test('calculateHistoricalBalances writes missing balances in one query', function () {
+    $user = User::factory()->onboarded()->create();
+    $connection = BankingConnection::factory()->create(['user_id' => $user->id]);
+    $account = Account::factory()->connected()->create([
+        'user_id' => $user->id,
+        'banking_connection_id' => $connection->id,
+        'external_account_id' => 'ext-123',
+    ]);
+
+    AccountBalance::factory()->create([
+        'account_id' => $account->id,
+        'balance_date' => '2026-02-10',
+        'balance' => 100000,
+    ]);
+
+    foreach (range(0, 5) as $daysBack) {
+        Transaction::factory()->enableBanking()->create([
+            'user_id' => $user->id,
+            'account_id' => $account->id,
+            'transaction_date' => sprintf('2026-02-%02d', 10 - $daysBack),
+            'amount' => -1000,
+        ]);
+    }
+
+    $service = new BalanceSyncService(Mockery::mock(BankingProviderInterface::class));
+
+    $result = countQueries(fn () => $service->calculateHistoricalBalances($account));
+    $balanceWrites = collect($result['queries'])->filter(fn (string $query): bool => str_contains($query, 'account_balances') && str_contains(strtolower($query), 'insert'));
+
+    expect($balanceWrites)->toHaveCount(1)
+        ->and($account->balances()->count())->toBe(6);
+});
+
 test('calculateHistoricalBalances handles multiple transactions per day', function () {
     $user = User::factory()->onboarded()->create();
     $connection = BankingConnection::factory()->create(['user_id' => $user->id]);