feat(board): master-task board, non-destructive stall watchdog, review flow

Board is now a clean master-task view:
- GetBoardAsync returns only top-level (master) tasks; child-tasks render
  nested inside their parent card instead of as separate column cards, so a
  big task split into many sub-tasks stays one card (orphans treated as master)
- New DoneChildTaskCount on the DTO for real progress bars
- Child/detail consumers (GetChildren endpoint, TaskBridgeService) query
  children directly instead of scraping the flat board

Stall watchdog (replaces destructive auto-reset):
- StaleTaskRecoveryService.FlagStalledInProgressTasksAsync marks In-progress
  tasks with no activity past the threshold as stalled (activity event +
  Iris notification) WITHOUT resetting the column — no work is discarded.
  Idempotent: a task is not re-flagged until real progress happens
- BackgroundService now runs this watchdog (TaskRecovery:StalledMinutes=40,
  interval 10m); hard reset kept only on the explicit manual endpoint

Review flow (Bao/Iris only):
- POST tasks/{id}/approve (Review -> Done)
- POST tasks/{id}/request-changes (Review -> target, mandatory comment,
  ExpectedFrom=iris, notifies Iris)

Frontend:
- BoardCard component: master card with ball chip (who has it), progress from
  children, expand to show children grouped by agent with per-child state +
  stalled marker, stalled chip on the master, review action buttons
- Request-changes modal; tasks store approveReview/requestChanges actions

Tests: watchdog flag/idempotency + review threshold; 135 backend tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/Services/IStaleTaskRecoveryService.cs

@@ -2,5 +2,9 @@ namespace Nexus.Api.Services;
 
 public interface IStaleTaskRecoveryService
 {
+    /// <summary>Nicht-destruktiv: markiert hängende In-progress-Tasks und benachrichtigt Iris.</summary>
+    Task<int> FlagStalledInProgressTasksAsync(TimeSpan stalledThreshold, CancellationToken ct = default);
+
+    /// <summary>Destruktiv (nur manuell): setzt hängende In-progress-Tasks hart auf Backlog.</summary>
     Task<int> ResetStaleInProgressTasksAsync(TimeSpan staleThreshold, CancellationToken ct = default);
 }