Add upcoming lecture calendar subscription

[msaroufim]

Summary

• publish upcoming Discord lectures as a standards-compliant iCalendar feed at /api/events/calendar.ics
• preserve stable Discord-backed event IDs so Google Calendar can update existing lectures instead of creating duplicates
• add a one-click Subscribe in Google Calendar button to the Upcoming Lectures section
• keep subscribed lectures transparent so following the calendar does not automatically block work availability

Why

People currently need to revisit gpumode.com or add individual Discord events. This provides a one-time subscription that continues surfacing new lectures and changes in their Google Calendar.

Validation

• uv run pytest tests/api/test_events_api.py -q
• uv run ruff check kernelboard/ tests/
• npm run lint
• npm run build