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/models/task.py

@@ -43,3 +43,7 @@ class Task(Base):
     assignee = relationship("User", foreign_keys=[assignee_id], back_populates="assigned_tasks")
     creator = relationship("User", foreign_keys=[created_by], back_populates="created_tasks")
     status = relationship("TaskStatus", back_populates="tasks")
+
+    # Collaboration relationships
+    comments = relationship("Comment", back_populates="task", cascade="all, delete-orphan")
+    blockers = relationship("Blocker", back_populates="task", cascade="all, delete-orphan")