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/comment.py

@@ -0,0 +1,52 @@
+from datetime import datetime
+from typing import Optional, List
+from pydantic import BaseModel, Field
+
+
+class CommentCreate(BaseModel):
+    content: str = Field(..., min_length=1, max_length=10000)
+    parent_comment_id: Optional[str] = None
+
+
+class CommentUpdate(BaseModel):
+    content: str = Field(..., min_length=1, max_length=10000)
+
+
+class CommentAuthor(BaseModel):
+    id: str
+    name: str
+    email: str
+
+    class Config:
+        from_attributes = True
+
+
+class MentionedUser(BaseModel):
+    id: str
+    name: str
+    email: str
+
+    class Config:
+        from_attributes = True
+
+
+class CommentResponse(BaseModel):
+    id: str
+    task_id: str
+    parent_comment_id: Optional[str]
+    content: str
+    is_edited: bool
+    is_deleted: bool
+    created_at: datetime
+    updated_at: datetime
+    author: CommentAuthor
+    mentions: List[MentionedUser] = []
+    reply_count: int = 0
+
+    class Config:
+        from_attributes = True
+
+
+class CommentListResponse(BaseModel):
+    comments: List[CommentResponse]
+    total: int