Implement Multi-Currency Support

[FerdinandSu]

Description / 描述

English:
Add support for multiple currencies:

• Store currency code with each entry (ISO 4217 codes)
• Add currency conversion rates table
• Allow viewing reports in different currencies
• Support currency conversion in views

Changes needed:

• Add currency field to Entry model
• Create CurrencyRate model for exchange rates
• Update views to support currency conversion
• Add endpoints to manage currency rates

中文:
添加多货币支持：

• 在每个条目中存储货币代码（ISO 4217 代码）
• 添加货币转换汇率表
• 允许以不同货币查看报告
• 在视图中支持货币转换

需要的更改：

• 向 Entry 模型添加 currency 字段
• 创建 CurrencyRate 模型用于汇率
• 更新视图以支持货币转换
• 添加管理货币汇率的端点

Difficulty / 难度

Hard / 困难

Estimated Time / 预计时间

10-14 hours

Acceptance Criteria / 验收标准

• Design database schema for currencies
• Add migration to add currency field to entries
• Implement currency conversion logic
• Create currency management endpoints
• Update all views to support currency conversion
• Add unit tests for currency conversion
• Update documentation

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