fix(www): stop on-demand mdx events from winning the events marquee (#48264)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Bug fix — the [events index](https://supabase.com/events) was featuring
the TRAE webinar in its marquee days after the event happened, instead
of the next genuinely upcoming event (the Dublin meetup), and counting
it toward the "Webinar" filter chip alongside actually-upcoming
webinars.

## What is the current behavior?

`getMdxEvents()` in `lib/events.ts` only excludes past events by
comparing dates against the start of today in UTC. It doesn't check
`onDemand` at all. The TRAE event's timestamp
(`2026-07-22T19:00:00.000-07:00`) converts to `2026-07-23T02:00:00Z`,
which is still "today or later" by that UTC cutoff — so even though the
event already happened and flipped to `onDemand: true`, it kept getting
returned as an "upcoming" event. Since the events marquee
(`featuredEvent`) just picks the earliest-dated event from that pool,
TRAE kept winning over the actually-upcoming Dublin meetup, and it kept
counting toward the "Webinar" filter chip.

## What is the new behavior?

`getMdxEvents()` now excludes any event with `onDemand: true` outright,
regardless of how its date converts across timezones — on-demand events
belong solely in the on-demand bucket (`getOnDemandMdxEvents`), not the
upcoming/marquee pool. Verified locally: the "Webinar" filter chip count
on `/events` drops to 0 with this in place (previously counted TRAE),
while the TRAE event's card in the on-demand list still correctly shows
its "Webinar" tag and "Supabase Live" line, matching the other on-demand
webinars (Perplexity, Datadog) — only its bucket assignment changed, not
its labeling.

## Additional context

Couldn't verify the marquee itself locally since the Luma events API
returns a 500 in local dev (missing credentials, pre-existing/unrelated
to this change). Confirmed independently via the production Luma API
that the Dublin meetup (`2026-07-28T17:00:00Z`) is genuinely the next
chronological event, so it will naturally take over the marquee once
this ships — no hardcoding needed.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
  * On-demand events are no longer shown in the upcoming events list.
* Upcoming events continue to be filtered by their relevant date, while
on-demand event listings remain unchanged.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/www/lib/events.ts

@@ -269,10 +269,15 @@ function getAllParsedMdxEvents(): SupabaseEvent[] {
 /**
  * Read all events under `_events/` and return today-and-future events.
  * Past events are excluded (by end_date when present, otherwise start date).
+ * Events already flipped to `onDemand: true` are excluded too — they've
+ * already happened and belong in the on-demand bucket (getOnDemandMdxEvents),
+ * not the upcoming one, regardless of how their UTC-converted date compares to today.
  */
 export const getMdxEvents = (): SupabaseEvent[] => {
   const today = startOfTodayUtc()
-  return getAllParsedMdxEvents().filter((event) => new Date(event.end_date ?? event.date) >= today)
+  return getAllParsedMdxEvents().filter(
+    (event) => !event.onDemand && new Date(event.end_date ?? event.date) >= today
+  )
 }
 
 /** All MDX events with `onDemand: true`, including past recordings. */