chore: TixologiWidget.concerts schema documented

Fields observed from live inspection:

• Concert ID field: array index (0-indexed) and data-sgf-concert HTML attribute
• Tixologi event ID field: eventId (integer)
• Lottery open field: open (ISO 8601 datetime string)
• Lottery close field: close (ISO 8601 datetime string)
• Show date field: dateMatch (partial date string, e.g. 'may 3')
• Artist name field: NOT in widget (must scrape from HTML)