fix(events): midnight in an announcement is 12:00 am on the Node we ship

`startsAtLabel` asked for `hour12: true` on `en-GB`. That is not the same
request as a 12-hour clock, and it does not survive a Node upgrade: for a
locale whose default cycle is h23, Node 20 resolves `hour12: true` to h11,
whose hours run 0-11, so midnight renders "0:00 am". Node 22 and later
resolve it to h12 and it renders "12:00 am". Same ICU on both sides, so it
is V8's ECMA-402 behaviour rather than locale data.

The image ships node:20-alpine and CI runs Node 20, while a dev machine is
newer -- which is how this rendered correctly in front of everyone who wrote
it and wrongly for every real recipient. An event mail announcing a midnight
start said "0:00 am" while the schedule editor beside it said "12:00 AM":
one instant, two spellings, which is the exact contradiction the option was
added to prevent.

`hourCycle: 'h12'` is the request that means what was meant. `recurrence.js`
already states the mirror-image rule for `h23`, and every other formatter in
this repo and in module-uo uses `hourCycle`; there is no `hour12` left here.

This is the one test that has been red on every events PR since #192, and
the only one -- each of those runs reported `# fail 1`. Verified by running
the suite under node:20-alpine, where the test fails without this change and
2152 tests pass with it; on Node 22+ it passes either way, so the test's
comment now says that a green run on a dev machine is not evidence.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

server/src/events/announce.js

@@ -67,7 +67,18 @@ function startsAtLabel(at, zone) {
       // Explicit rather than left to the locale, because `en-GB` would otherwise
       // render midnight as "00:00" while the schedule editor beside it writes
       // "12:00 AM" — one event, two spellings of the same instant.
-      hour12: true,
+      //
+      // `hourCycle: 'h12'` and NOT `hour12: true`, which is not the same request
+      // and does not survive a Node upgrade. For a locale whose default cycle is
+      // h23 — `en-GB` is one — Node 20 resolves `hour12: true` to **h11**, whose
+      // hours run 0–11, so midnight comes out "0:00 am"; Node 22 and later
+      // resolve it to h12 and it comes out "12:00 am". Same ICU on both, so this
+      // is V8's ECMA-402 behaviour and not locale data, and the image ships
+      // node:20-alpine while a dev machine is newer — which is how this rendered
+      // correctly in front of everyone who wrote it and wrongly for every real
+      // recipient. `recurrence.js` states the mirror-image rule for `h23`; there
+      // is no `hour12` left in this repo and it should stay that way.
+      hourCycle: 'h12',
     }).format(when)
     return `${text} (${zone || 'UTC'})`
   } catch {