Add Data Export Functionality (CSV/Excel)

[FerdinandSu]

Description / 描述

English:
Users should be able to export their ledger data in common formats:

• CSV export for entries, categories, and views
• Excel (XLSX) export with multiple sheets
• Support date range and category filters
• Include summary statistics in exports

Add new endpoints:

• GET /ledger/export/csv
• GET /ledger/export/excel

中文:
用户应该能够以常见格式导出账本数据：

• 导出条目、类别和视图的 CSV
• 带多个工作表的 Excel (XLSX) 导出
• 支持日期范围和类别过滤器
• 在导出中包含汇总统计信息

添加新的端点：

• GET /ledger/export/csv
• GET /ledger/export/excel

Difficulty / 难度

Medium-Hard / 中等-困难

Estimated Time / 预计时间

6-8 hours

Acceptance Criteria / 验收标准

• Install required packages (EPPlus or similar)
• Implement CSV export endpoint
• Implement Excel export endpoint
• Support query parameters for filtering
• Add proper content-type headers and file naming
• Update API documentation
• Add integration tests

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