fix: [bank-account] 계좌 관리 API 누락 필드 8개 보강

- account_type, balance, currency, opened_at, branch_name, memo, sort_order, last_transaction_at 추가
- Model: fillable, casts, hidden, scopes, accessors를 MNG 모델 기준으로 통일
- Service: store/update에 누락 필드 반영
- FormRequest: Store/Update에 검증 규칙 추가

app/Http/Requests/V1/BankAccount/StoreBankAccountRequest.php

@@ -19,9 +19,16 @@ public function rules(): array
             'account_number' => ['required', 'string', 'max:30', 'regex:/^[\d-]+$/'],
             'account_holder' => ['required', 'string', 'max:50'],
             'account_name' => ['required', 'string', 'max:100'],
+            'account_type' => ['nullable', 'string', 'max:30'],
+            'balance' => ['nullable', 'numeric', 'min:0'],
+            'currency' => ['nullable', 'string', 'max:3'],
+            'opened_at' => ['nullable', 'date'],
+            'branch_name' => ['nullable', 'string', 'max:100'],
+            'memo' => ['nullable', 'string', 'max:500'],
             'status' => ['nullable', 'string', 'in:active,inactive'],
             'assigned_user_id' => ['nullable', 'integer', 'exists:users,id'],
             'is_primary' => ['nullable', 'boolean'],
+            'sort_order' => ['nullable', 'integer', 'min:0'],
         ];
     }