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/main.py

@@ -8,6 +8,10 @@ from app.api.spaces import router as spaces_router
 from app.api.projects import router as projects_router
 from app.api.tasks import router as tasks_router
 from app.api.workload import router as workload_router
+from app.api.comments import router as comments_router
+from app.api.notifications import router as notifications_router
+from app.api.blockers import router as blockers_router
+from app.api.websocket import router as websocket_router
 from app.core.config import settings
 
 app = FastAPI(
@@ -33,6 +37,10 @@ app.include_router(spaces_router)
 app.include_router(projects_router)
 app.include_router(tasks_router)
 app.include_router(workload_router, prefix="/api/workload", tags=["Workload"])
+app.include_router(comments_router)
+app.include_router(notifications_router)
+app.include_router(blockers_router)
+app.include_router(websocket_router)
 
 
 @app.get("/health")