Group separate events (Camp 2025, Camp 2026) on a series page at
/series/<slug> with upcoming and past events. Events join a series from
the event editor (copies keep it); a series can span campuses. Event
pages show "Part of ..." and "Next in this series". Signed-in people can
follow a series; the new `series` job emails followers once when a new
event in it goes live, as News & announcements (opted-in only, with
unsubscribe). Follows are in the data export and removed on
anonymising. Migration 20261028140000_event_series.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
joshuaandClaude Opus 5.5
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