fix: use half-open date ranges for DST-safe usage queries

Replace t.Add(24*time.Hour - time.Nanosecond) with t.AddDate(0, 0, 1) and use SQL < instead of <= for end-of-day boundaries. This avoids edge-case misses around DST transitions.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/handler/admin/usage_handler.go

@@ -159,8 +159,8 @@ func (h *UsageHandler) List(c *gin.Context) {
 			response.BadRequest(c, "Invalid end_date format, use YYYY-MM-DD")
 			return
 		}
-		// Set end time to end of day
-		t = t.Add(24*time.Hour - time.Nanosecond)
+		// Use half-open range [start, end), move to next calendar day start (DST-safe).
+		t = t.AddDate(0, 0, 1)
 		endTime = &t
 	}
 
@@ -285,7 +285,8 @@ func (h *UsageHandler) Stats(c *gin.Context) {
 			response.BadRequest(c, "Invalid end_date format, use YYYY-MM-DD")
 			return
 		}
-		endTime = endTime.Add(24*time.Hour - time.Nanosecond)
+		// 与 SQL 条件 created_at < end 对齐，使用次日 00:00 作为上边界（DST-safe）。
+		endTime = endTime.AddDate(0, 0, 1)
 	} else {
 		period := c.DefaultQuery("period", "today")
 		switch period {

backend/internal/handler/usage_handler.go

@@ -114,8 +114,8 @@ func (h *UsageHandler) List(c *gin.Context) {
 			response.BadRequest(c, "Invalid end_date format, use YYYY-MM-DD")
 			return
 		}
-		// Set end time to end of day
-		t = t.Add(24*time.Hour - time.Nanosecond)
+		// Use half-open range [start, end), move to next calendar day start (DST-safe).
+		t = t.AddDate(0, 0, 1)
 		endTime = &t
 	}
 
@@ -227,8 +227,8 @@ func (h *UsageHandler) Stats(c *gin.Context) {
 			response.BadRequest(c, "Invalid end_date format, use YYYY-MM-DD")
 			return
 		}
-		// 设置结束时间为当天结束
-		endTime = endTime.Add(24*time.Hour - time.Nanosecond)
+		// 与 SQL 条件 created_at < end 对齐，使用次日 00:00 作为上边界（DST-safe）。
+		endTime = endTime.AddDate(0, 0, 1)
 	} else {
 		// 使用 period 参数
 		period := c.DefaultQuery("period", "today")

backend/internal/repository/usage_log_repo.go

@@ -3004,7 +3004,7 @@ func (r *usageLogRepository) GetGlobalStats(ctx context.Context, startTime, endT
 			COALESCE(SUM(actual_cost), 0) as total_actual_cost,
 			COALESCE(AVG(duration_ms), 0) as avg_duration_ms
 		FROM usage_logs
-		WHERE created_at >= $1 AND created_at <= $2
+		WHERE created_at >= $1 AND created_at < $2
 	`
 
 	stats := &UsageStats{}