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>

openspec/changes/archive/2025-12-29-add-collaboration/proposal.md

@@ -0,0 +1,18 @@
+# Change: Add Collaboration Features
+
+## Why
+專案管理系統需要協作功能，讓團隊成員能在任務內進行討論、標記相關人員，並處理任務阻礙。目前系統缺乏這些核心協作機制，導致使用者仍需依賴 Email 或其他工具進行溝通。
+
+## What Changes
+- 新增任務留言系統（支援巢狀回覆）
+- 實作 @提及功能（自動完成 + 即時通知）
+- 建立阻礙管理機制（標記原因、主管通知、解除追蹤）
+- 實作即時通知系統（Redis Pub/Sub + WebSocket）
+
+## Impact
+- Affected specs: `collaboration`
+- Affected code:
+  - 新增 models: Comment, Mention, Notification, Blocker
+  - 新增 API routes: `/comments`, `/notifications`
+  - 新增 services: NotificationService, WebSocketManager
+  - 前端: 任務詳情頁新增留言區、通知中心