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/schemas/notification.py

@@ -0,0 +1,28 @@
+from datetime import datetime
+from typing import Optional, List
+from pydantic import BaseModel
+
+
+class NotificationResponse(BaseModel):
+    id: str
+    type: str
+    reference_type: str
+    reference_id: str
+    title: str
+    message: Optional[str]
+    is_read: bool
+    created_at: datetime
+    read_at: Optional[datetime]
+
+    class Config:
+        from_attributes = True
+
+
+class NotificationListResponse(BaseModel):
+    notifications: List[NotificationResponse]
+    total: int
+    unread_count: int
+
+
+class UnreadCountResponse(BaseModel):
+    unread_count: int