Add Budget Tracking Feature

[FerdinandSu]

Description / 描述

English:
Implement a budget tracking system that allows users to:

• Set monthly/yearly budgets for categories
• Track spending against budgets
• Get alerts when approaching budget limits
• View budget vs actual reports

New models needed:

• Budget (category, amount, period, startDate, endDate)
• BudgetAlert (threshold percentage)

New endpoints:

• POST /ledger/budget - Create budget
• GET /ledger/budgets - List budgets
• GET /ledger/budget/{id}/status - Get budget status
• DELETE /ledger/budget/{id} - Delete budget

中文:
实现预算跟踪系统，允许用户：

• 为类别设置月度/年度预算
• 跟踪预算支出
• 在接近预算限制时获取提醒
• 查看预算与实际报告

Difficulty / 难度

Hard / 困难

Estimated Time / 预计时间

12-16 hours

Acceptance Criteria / 验收标准

• Design database schema for budgets
• Create EF Core migrations
• Implement IBudgetManager interface
• Create BudgetController with all endpoints
• Add budget calculation logic
• Update Swagger documentation
• Write unit and integration tests
• Add documentation with examples

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