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/user.py

@@ -29,3 +29,10 @@ class User(Base):
     owned_projects = relationship("Project", foreign_keys="Project.owner_id", back_populates="owner")
     assigned_tasks = relationship("Task", foreign_keys="Task.assignee_id", back_populates="assignee")
     created_tasks = relationship("Task", foreign_keys="Task.created_by", back_populates="creator")
+
+    # Collaboration relationships
+    comments = relationship("Comment", back_populates="author")
+    mentions = relationship("Mention", back_populates="mentioned_user")
+    notifications = relationship("Notification", back_populates="user", cascade="all, delete-orphan")
+    reported_blockers = relationship("Blocker", foreign_keys="Blocker.reported_by", back_populates="reporter")
+    resolved_blockers = relationship("Blocker", foreign_keys="Blocker.resolved_by", back_populates="resolver")