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/app/models/__init__.py

@@ -6,5 +6,12 @@ from app.models.project import Project
 from app.models.task_status import TaskStatus
 from app.models.task import Task
 from app.models.workload_snapshot import WorkloadSnapshot
+from app.models.comment import Comment
+from app.models.mention import Mention
+from app.models.notification import Notification
+from app.models.blocker import Blocker
 
-__all__ = ["User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot"]
+__all__ = [
+    "User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot",
+    "Comment", "Mention", "Notification", "Blocker"
+]