feat(worker): Online Inbox sync engine (Phase 1)

Optional, opt-in (online_inbox.enabled, default false → zero network).
Worker-side reconcile loop: pull web-created tasks down as Idle, push the
list catalog and the Idle backlog mirror up. Auth behind IOnlineAuthProvider
(StaticTokenAuthProvider default; ZitadelAuthProvider stubbed for Phase 2).
DPAPI refresh-token store. 35 tests, no real network/Zitadel/Claude.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -87,6 +87,22 @@ public sealed class TaskRepository
             .ToListAsync(ct);
     }
 
+    /// <summary>
+    /// Returns all tasks that qualify as "real" Idle backlog items for online mirroring:
+    /// Status==Idle, no parent, PlanningPhase==None, not blocked.
+    /// </summary>
+    public async Task<List<TaskEntity>> GetAllIdleBacklogAsync(CancellationToken ct = default)
+    {
+        return await _context.Tasks
+            .AsNoTracking()
+            .Where(t => t.Status == TaskStatus.Idle
+                        && t.ParentTaskId == null
+                        && t.PlanningPhase == PlanningPhase.None
+                        && t.BlockedByTaskId == null)
+            .OrderBy(t => t.SortOrder).ThenBy(t => t.CreatedAt)
+            .ToListAsync(ct);
+    }
+
     #endregion
 
     #region Status transitions