feat: add tiered storage guard (Green/Yellow/Red) and dashboard queue monitor

Storage Tier System:
- Add StorageTier enum (Green >30% / Yellow 10-30% / Red <10%)
- Extend StorageGuardResult with Tier, CanStartNewRecording, ShouldPauseActive, UsagePercent
- Yellow tier: deny new recordings but allow existing to finish and upload
- Red tier: deny new recordings and pause active sessions
- Auto-recovery: when disk frees up, polling automatically resumes new recordings
- Update LiveRoomPollingBackgroundService to use tier-based checks
- Expose tier + usage percent in Recovery API

Dashboard Queue Monitor:
- Add pending transcode count, pending upload count, queued data volume to dashboard
- Add storage tier badge (Green/Yellow/Red) with usage percentage
- Add queue monitoring card to dashboard view

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

src/LiveRecorder.Application/Services/DashboardService.cs

@@ -90,8 +90,13 @@ public sealed class DashboardService
             {
                 HasEnoughSpace = storageCheck.HasEnoughSpace,
                 Message = storageCheck.Message ?? string.Empty,
-                AvailableBytes = storageCheck.AvailableBytes
+                AvailableBytes = storageCheck.AvailableBytes,
+                Tier = storageCheck.Tier.ToString(),
+                UsagePercent = storageCheck.UsagePercent
             },
+            PendingTranscodeCount = await _recordTaskRepository.CountByStatusAsync(Domain.Enums.RecordTaskStatus.Processing, cancellationToken),
+            PendingUploadCount = await _recordResultRepository.CountPendingUploadAsync(cancellationToken),
+            QueuedDataBytes = await _recordResultRepository.SumPendingUploadBytesAsync(cancellationToken),
             RecentSessions = recentSessionsTask.Result
                 .Select(MapRecentSession)
                 .ToList(),