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/__init__.py

@@ -11,6 +11,15 @@ from app.schemas.task import (
     TaskCreate, TaskUpdate, TaskResponse, TaskWithDetails, TaskListResponse,
     TaskStatusUpdate as TaskStatusChangeUpdate, TaskAssignUpdate, Priority
 )
+from app.schemas.comment import (
+    CommentCreate, CommentUpdate, CommentResponse, CommentListResponse
+)
+from app.schemas.notification import (
+    NotificationResponse, NotificationListResponse, UnreadCountResponse
+)
+from app.schemas.blocker import (
+    BlockerCreate, BlockerResolve, BlockerResponse, BlockerListResponse
+)
 
 __all__ = [
     "LoginRequest",
@@ -44,4 +53,15 @@ __all__ = [
     "TaskStatusChangeUpdate",
     "TaskAssignUpdate",
     "Priority",
+    "CommentCreate",
+    "CommentUpdate",
+    "CommentResponse",
+    "CommentListResponse",
+    "NotificationResponse",
+    "NotificationListResponse",
+    "UnreadCountResponse",
+    "BlockerCreate",
+    "BlockerResolve",
+    "BlockerResponse",
+    "BlockerListResponse",
 ]