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/api/notifications/__init__.py

@@ -0,0 +1,3 @@
+from app.api.notifications.router import router
+
+__all__ = ["router"]

backend/app/api/notifications/router.py

@@ -0,0 +1,119 @@
+from typing import Optional
+from datetime import datetime
+from fastapi import APIRouter, Depends, HTTPException, status, Query
+from sqlalchemy.orm import Session
+
+from app.core.database import get_db
+from app.models import User, Notification
+from app.schemas.notification import (
+    NotificationResponse, NotificationListResponse, UnreadCountResponse
+)
+from app.middleware.auth import get_current_user
+
+router = APIRouter(tags=["notifications"])
+
+
+def notification_to_response(notification: Notification) -> NotificationResponse:
+    """Convert Notification model to NotificationResponse."""
+    return NotificationResponse(
+        id=notification.id,
+        type=notification.type,
+        reference_type=notification.reference_type,
+        reference_id=notification.reference_id,
+        title=notification.title,
+        message=notification.message,
+        is_read=notification.is_read,
+        created_at=notification.created_at,
+        read_at=notification.read_at,
+    )
+
+
+@router.get("/api/notifications", response_model=NotificationListResponse)
+async def list_notifications(
+    is_read: Optional[bool] = Query(None, description="Filter by read status"),
+    limit: int = Query(50, ge=1, le=100, description="Number of notifications to return"),
+    offset: int = Query(0, ge=0, description="Offset for pagination"),
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """List notifications for the current user."""
+    query = db.query(Notification).filter(Notification.user_id == current_user.id)
+
+    if is_read is not None:
+        query = query.filter(Notification.is_read == is_read)
+
+    total = query.count()
+    unread_count = db.query(Notification).filter(
+        Notification.user_id == current_user.id,
+        Notification.is_read == False,
+    ).count()
+
+    notifications = query.order_by(Notification.created_at.desc()).offset(offset).limit(limit).all()
+
+    return NotificationListResponse(
+        notifications=[notification_to_response(n) for n in notifications],
+        total=total,
+        unread_count=unread_count,
+    )
+
+
+@router.get("/api/notifications/unread-count", response_model=UnreadCountResponse)
+async def get_unread_count(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Get count of unread notifications."""
+    count = db.query(Notification).filter(
+        Notification.user_id == current_user.id,
+        Notification.is_read == False,
+    ).count()
+
+    return UnreadCountResponse(unread_count=count)
+
+
+@router.put("/api/notifications/{notification_id}/read", response_model=NotificationResponse)
+async def mark_as_read(
+    notification_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Mark a notification as read."""
+    notification = db.query(Notification).filter(
+        Notification.id == notification_id,
+        Notification.user_id == current_user.id,
+    ).first()
+
+    if not notification:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Notification not found",
+        )
+
+    if not notification.is_read:
+        notification.is_read = True
+        notification.read_at = datetime.utcnow()
+        db.commit()
+        db.refresh(notification)
+
+    return notification_to_response(notification)
+
+
+@router.put("/api/notifications/read-all", response_model=dict)
+async def mark_all_as_read(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Mark all notifications as read."""
+    now = datetime.utcnow()
+
+    updated_count = db.query(Notification).filter(
+        Notification.user_id == current_user.id,
+        Notification.is_read == False,
+    ).update({
+        Notification.is_read: True,
+        Notification.read_at: now,
+    })
+
+    db.commit()
+
+    return {"updated_count": updated_count}