feat: implement collaboration module

- Backend (FastAPI):
  - Task comments with nested replies and soft delete
  - @mention parsing with 10-mention limit per comment
  - Notification system with read/unread tracking
  - Blocker management with project owner notification
  - WebSocket endpoint with JWT auth and keepalive
  - User search API for @mention autocomplete
  - Alembic migration for 4 new tables

- Frontend (React + Vite):
  - Comments component with @mention autocomplete
  - NotificationBell with real-time WebSocket updates
  - BlockerDialog for task blocking workflow
  - NotificationContext for state management

- OpenSpec:
  - 4 requirements with scenarios defined
  - add-collaboration change archived

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

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

backend/migrations/env.py

@@ -10,7 +10,10 @@ sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 from app.core.config import settings
 from app.core.database import Base
-from app.models import User, Role, Department
+from app.models import (
+    User, Role, Department, Space, Project, TaskStatus, Task, WorkloadSnapshot,
+    Comment, Mention, Notification, Blocker
+)
 
 config = context.config