feat: 添加图表统计功能

- TG: /chart 本月分类饼图, /week 近7天消费柱状图
- QQ: 统计/报表 本月文本统计
- 新增 go-chart 依赖生成 PNG 图表
- 新增 GetCategoryStats/GetDailyStats 查询方法

internal/service/finance.go

@@ -114,3 +114,41 @@ func (s *FinanceService) GetTransactionsByDate(userID int64, date string) ([]mod
 		Order("id desc").Find(&items).Error
 	return items, err
 }
+
+// CategoryStat 分类统计结果
+type CategoryStat struct {
+	Category string
+	Total    int64
+	Count    int
+}
+
+// GetCategoryStats 获取用户指定日期范围的分类统计
+func (s *FinanceService) GetCategoryStats(userID int64, dateFrom, dateTo string) ([]CategoryStat, error) {
+	var stats []CategoryStat
+	err := s.db.Model(&models.Transaction{}).
+		Select("category, SUM(amount) as total, COUNT(*) as count").
+		Where("user_id = ? AND date >= ? AND date <= ? AND is_deleted = ?", userID, dateFrom, dateTo, false).
+		Group("category").
+		Order("total desc").
+		Find(&stats).Error
+	return stats, err
+}
+
+// DailyStat 每日统计结果
+type DailyStat struct {
+	Date  string
+	Total int64
+	Count int
+}
+
+// GetDailyStats 获取用户指定日期范围的每日统计
+func (s *FinanceService) GetDailyStats(userID int64, dateFrom, dateTo string) ([]DailyStat, error) {
+	var stats []DailyStat
+	err := s.db.Model(&models.Transaction{}).
+		Select("date, SUM(amount) as total, COUNT(*) as count").
+		Where("user_id = ? AND date >= ? AND date <= ? AND is_deleted = ?", userID, dateFrom, dateTo, false).
+		Group("date").
+		Order("date asc").
+		Find(&stats).Error
+	return stats, err
+}