fix: only show live contests on scoreboard, hide ended ones

Filter out ended contests from the /contests/active endpoint so the
scoreboard only displays running or upcoming contests. When all contests
have ended, the frontend shows the existing "No Active Contests" empty
state instead of stale scoreboards.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Uttam-Mahata

backend/internal/handlers/scoreboard_handler.go

@@ -194,13 +194,18 @@ func (h *ScoreboardHandler) GetScoreboardContests(c *gin.Context) {
 		if contest.GetScoreboardVisibility() == "hidden" {
 			continue
 		}
+		// Only show live (running or upcoming) contests on the scoreboard
+		status := contest.Status(now)
+		if status == "ended" {
+			continue
+		}
 		result = append(result, gin.H{
 			"id":                    contest.ID,
 			"name":                  contest.Name,
 			"description":           contest.Description,
 			"start_time":            contest.StartTime,
 			"end_time":              contest.EndTime,
-			"status":                contest.Status(now),
+			"status":                status,
 			"scoreboard_visibility": contest.GetScoreboardVisibility(),
 		})
 	}