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/TaskBridgeService.cs

@@ -214,13 +214,7 @@ public sealed class TaskBridgeService(
 
     public async Task<IReadOnlyList<DashboardTaskDto>> GetChildTasksAsync(
         Guid parentTaskId, CancellationToken ct = default)
-    {
-        var board = await taskService.GetBoardAsync(ct);
-        return FlattenBoard(board)
-            .Where(task => task.ParentTaskId == parentTaskId)
-            .OrderByDescending(task => task.UpdatedAt)
-            .ToList();
-    }
+        => await taskService.GetChildTaskDtosAsync(parentTaskId, ct);
 
     public async Task<List<ActivityEvent>> GetTaskActivityAsync(
         Guid taskId, CancellationToken ct = default)
@@ -250,13 +244,6 @@ public sealed class TaskBridgeService(
         return AgentIdentityCatalog.NormalizeActorId(actorId, allowedActors);
     }
 
-    private static IEnumerable<DashboardTaskDto> FlattenBoard(BoardResponse board)
-        => board.Offen
-            .Concat(board.InProgress)
-            .Concat(board.Review)
-            .Concat(board.Blocked)
-            .Concat(board.Done);
-
     private static DashboardTaskDto MapToDto(WorkTask t) => new(
         t.Id, t.Title, t.Detail, t.Source, t.State, t.Priority, t.AssignedTo,
         t.ParentTaskId, t.DueDate, t.CreatedAt, t.UpdatedAt,