feat: implement kanban real-time sync and fix workload cache

## Kanban Real-time Sync (NEW-002)
- Backend:
  - WebSocket endpoint: /ws/projects/{project_id}
  - Project room management in ConnectionManager
  - Redis Pub/Sub: project:{project_id}:tasks channel
  - Task CRUD event publishing (5 event types)
  - Redis connection retry with exponential backoff
  - Race condition fix in broadcast_to_project

- Frontend:
  - ProjectSyncContext for WebSocket management
  - Reconnection with exponential backoff (max 5 attempts)
  - Multi-tab event deduplication via event_id
  - Live/Offline connection indicator
  - Optimistic updates with rollback

- Spec:
  - collaboration spec: +1 requirement (Project Real-time Sync)
  - 7 new scenarios for real-time sync

## Workload Cache Fix (NEW-001)
- Added cache invalidation to all task endpoints:
  - create_task, update_task, update_task_status
  - delete_task, restore_task, assign_task
- Extended to clear heatmap cache as well

## OpenSpec Archive
- 2026-01-05-add-kanban-realtime-sync

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/workload_cache.py

@@ -155,9 +155,19 @@ def invalidate_user_workload_cache(user_id: str) -> None:
     """
     Invalidate all cached workload data for a user.
 
+    This clears:
+    1. User-specific workload cache (workload:user:{user_id}:*)
+    2. All heatmap caches (workload:heatmap:*) since heatmap aggregates may include this user
+
     Note: This uses pattern matching which may be slow for large datasets.
     For Phase 1, we rely on TTL expiration instead of active invalidation.
     """
-    pattern = f"workload:*:{user_id}:*"
-    for key in redis_client.scan_iter(match=pattern):
+    # Clear user-specific workload cache
+    user_pattern = f"workload:user:{user_id}:*"
+    for key in redis_client.scan_iter(match=user_pattern):
+        redis_client.delete(key)
+
+    # Clear all heatmap caches since they aggregate user data
+    heatmap_pattern = "workload:heatmap:*"
+    for key in redis_client.scan_iter(match=heatmap_pattern):
         redis_client.delete(key)