fix: consolidate storage tier with old MB thresholds into single tier system

- CanStartNewRecording now purely based on Tier==Green (was HasEnoughSpace||Green)
- PollingBackgroundService now uses ShouldPauseActive instead of MB-based CheckShouldPause
- Both pause and start checks consolidated into single guardCheck call
- Old MB pause/resume thresholds still work as secondary safety via hasEnoughSpace

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

src/LiveRecorder.Application/Abstractions/Storage/IStorageGuardService.cs

@@ -40,12 +40,14 @@ public sealed record StorageGuardResult(
     public double UsagePercent { get; init; }
 
     /// <summary>
-    /// True if new recordings can be started. False in Yellow and Red tiers.
+    /// True if new recordings can be started. Only true in Green tier.
+    /// This replaces the old binary HasEnoughSpace check — the tier system is the single source of truth.
     /// </summary>
-    public bool CanStartNewRecording => HasEnoughSpace || Tier == StorageTier.Green;
+    public bool CanStartNewRecording => Tier == StorageTier.Green;
 
     /// <summary>
     /// True if active recordings should be paused. Only true in Red tier.
+    /// This replaces the old MB-based CheckShouldPause — consolidated into the tier system.
     /// </summary>
     public bool ShouldPauseActive => Tier == StorageTier.Red;
 }