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",
 ]

backend/app/schemas/blocker.py

@@ -0,0 +1,39 @@
+from datetime import datetime
+from typing import Optional, List
+from pydantic import BaseModel, Field
+
+
+class BlockerCreate(BaseModel):
+    reason: str = Field(..., min_length=1, max_length=2000)
+
+
+class BlockerResolve(BaseModel):
+    resolution_note: str = Field(..., min_length=1, max_length=2000)
+
+
+class BlockerUserInfo(BaseModel):
+    id: str
+    name: str
+    email: str
+
+    class Config:
+        from_attributes = True
+
+
+class BlockerResponse(BaseModel):
+    id: str
+    task_id: str
+    reason: str
+    resolution_note: Optional[str]
+    created_at: datetime
+    resolved_at: Optional[datetime]
+    reporter: BlockerUserInfo
+    resolver: Optional[BlockerUserInfo]
+
+    class Config:
+        from_attributes = True
+
+
+class BlockerListResponse(BaseModel):
+    blockers: List[BlockerResponse]
+    total: int

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

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