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/mention.py

@@ -0,0 +1,18 @@
+import uuid
+from sqlalchemy import Column, String, DateTime, ForeignKey
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class Mention(Base):
+    __tablename__ = "pjctrl_mentions"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    comment_id = Column(String(36), ForeignKey("pjctrl_comments.id", ondelete="CASCADE"), nullable=False)
+    mentioned_user_id = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=False)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+
+    # Relationships
+    comment = relationship("Comment", back_populates="mentions")
+    mentioned_user = relationship("User", back_populates="mentions")