docs: update 8-14-expense-reports to match backend

[light-cmd]

Summary

Verified articles/08-expense-management/8-14-expense-reports.md against the backend monolith. The article described an expense analytics suite that does not exist in the product, so it has been rewritten to describe the actual feature.

Discrepancies found

The article documented the following features, none of which exist in the backend (no matching BFF resources, REST endpoints, or modules):

• Dashboard > Expense Summary and Executive Summary dashboards
• Reports > Spending by Category / by Employee / by Merchant / Spending Trends / Department Spending
• Policy Compliance, Reimbursement Metrics, Extraction Quality, and Cost Control Metrics reports
• Custom expense report builder, Scheduled Reports (emailed on a schedule), and Benchmarking against industry peers
• Excel/PDF export of expense reports

What the backend actually supports

• An expense report in Light is a batch of expenses submitted for reimbursement (ExpenseV1Resource POST /expenses/submit, ReimbursementDto.expenseIds)
• Employees track expenses on Expenses (/expenses); finance reviews via Reimbursements (/reimbursements, BffReimbursementV1Resource)
• Spend analysis happens via ledger Reports (/ledger-reports, LedgerReportV1Resource): Balance Sheet, Cash Flow Statement, General Ledger, Income Statement, Trial Balance, VAT (ReportDefinitionSystemName), plus pivot table reports
• CSV export exists (CsvExportV1Resource, GL report export); no Excel/PDF report export

Changes

• Rewrote the article to describe expense reports as reimbursement submissions, with pointers to ledger reports and CSV export for analysis
• Updated the 8.14 title in help-article-hierarchy.md accordingly