Implement real-time updates for task board and task chat

[ganeshwhere]

Objective

Provide near real-time collaboration updates for task board and task chat.

Scope

• Server push for task status/assignment changes
• Server push for new chat messages and mention events
• Client reconnect strategy with safe fallback behavior

Acceptance Criteria

• Connected collaborators see board updates within 1 second
• New chat messages appear without manual refresh
• Reconnect flow handles transient disconnects without data loss
• Fallback path works when realtime channel is unavailable

Non-Goals

• Mobile push notifications