fix(phase-4): close review notes — drop dead var, sync UI cron default

Phase 4 reviewer (Sonnet) flagged two 🟡 warnings; both addressed:

1. PullLiveOddsUseCase: removed dead 'liveEvents' assignment that called
   ScrapeUpcomingAsync but never read the result. Replaced misleading
   comment block with a single TODO(phase-6/8) note pointing to the
   ListLiveAsync(cutoff) follow-up.
2. Marathon.UI.Services.WorkerOptions.UpcomingScheduleCron: changed default
   from '0 */5 * * * *' (every 5 min) to '0 0 */6 * * *' (every 6 hours)
   to match Marathon.Infrastructure.Configuration.WorkerOptions and the
   appsettings.json default.

PLAN.md: Phase 4 row updated (review status, commit hash); top-level
checkbox in Phases list ticked.

Build 0/0, all 202 tests still passing.

src/Marathon.UI/Services/WorkerOptions.cs

@@ -9,7 +9,7 @@ public sealed class WorkerOptions
     public const string SectionName = "Workers";
 
     /// <summary>Cron expression that drives the upcoming-schedule poller.</summary>
-    public string UpcomingScheduleCron { get; set; } = "0 */5 * * * *";
+    public string UpcomingScheduleCron { get; set; } = "0 0 */6 * * *";
 
     /// <summary>Whether the live odds poller should run at startup.</summary>
     public bool LivePollerEnabled { get; set; } = true;