Filter past events out of the homepage carousel

[seandavi]

Fixes #401.

top_events() (lib/helpers.rb:452) sorted events by start date and took the last five, with no date filter — so the homepage carousel showed the five chronologically-latest events whether or not they had already happened. Its two siblings both filter correctly: upcoming_events() at :426 and previous_events() at :446 each test e[:end] against today.

This applies the same filter.

Why last(5) instead of keeping [-5..-1]

sorted[-5..-1] returns nil when the array holds fewer than five elements, and nil.reverse raises. That was latent before (it needed the site to have fewer than five events total) but the filter makes it reachable — any period with fewer than five upcoming events would hit it. last(5) returns what it has.

Verification

Ran the before/after logic against synthetic events (3 past, 2 upcoming) on Ruby 3.2:

old: ["later", "soon", "past3", "past2", "past1"]
new: ["later", "soon"]
sorted[-5..-1] with 2 elements -> nil
upcoming.last(5)               -> ["soon", "later"]

The old path leaks all three past events; the new one returns only upcoming, newest first, and the nil hazard is confirmed real rather than theoretical.

I was not able to run a full site build to check this end to end — no Ruby toolchain for this repo locally, and the pinned Ruby 2.6.5 / Debian Buster image is awkward to stand up. The change is confined to one function whose only caller is layouts/components/homepage/events.html, so the blast radius is the homepage carousel.

Note for reviewers

With the filter applied, the carousel will show fewer than five entries when fewer than five events are upcoming. That seemed clearly preferable to backfilling with finished events, but if the design wants exactly five slots filled, that's a different fix and worth saying so.

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