Lists: grid freshness poll and presence heartbeat #67

POST /api/lists/{id}/data/versions is a combined freshness poll and presence heartbeat for the list grid.

Acceptance criteria

• Request/response shape verified live before implementing.
• While a list is open, poll on a timer to detect rows changed by collaborators and refresh without losing in-progress edits.
• Presence (who else has the list open) shown if the response carries it.
• Polling stops when the view is not active; cadence checked against GET /api/limits and the general rate-limit posture.
• No blocking call on the WPF dispatcher (see the polling rule in CLAUDE.md).