MCP: add edit_poll tool

[zhiganov]

The MCP endpoint has create_poll but no way to edit an existing poll (update dates, time range, title, description). Had to resort to direct API calls to fix dates.

Should support at minimum: updating dates, timeRange, title, description, and slotMinutes.

[zhiganov]

Resolved by commit 5f14849 — the update_poll MCP tool is live and supports all fields listed here (title, description, dates, timeRange, slotMinutes, plus hideResponsesUntilSubmit). See server/src/mcp/tools.js.