chore(node): build for the runtime the image carries

`engines`, `@types/node` and the three esbuild targets said node20 while both
stages of the image have been node:24-alpine for a while — and since
24.10.2026 the development machines run 24 too. So the bundles are built for
exactly the runtime that carries them instead of for a version nobody runs
any more.

`npm run check`: 110 files, 1825 tests, green on node 24.22.1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/Dockerfile

@@ -11,11 +11,11 @@
 # and a .gz beside its files. The second stage takes only these files, so
 # the image holds no node_modules, no sources and no compiler.
 #
-# Node 24 (active LTS), not the 20 of the development machines: 20 has had
-# no security fixes since April 2026, and a service facing the browsers of a
-# whole school should not run on it. The bundles are built for node20 and
-# run unchanged on 24; the image's run is checked before each release (see
-# the README).
+# Node 24 (active LTS) — and since 24.10.2026 the same major as the
+# development machines, so the bundles are built for exactly the runtime that
+# carries them. 20 has had no security fixes since April 2026, and a service
+# facing the browsers of a whole school should not run on it. The image's run
+# is checked before each release (see the README).
 
 FROM node:24-alpine AS build
 WORKDIR /src