perf(nextjs): stop preloading every route's module graph at startup

Next loads the module graph for all 74 app routes when the server starts. Measured
here: 1,114 files and 24 MiB of source resident before the first request, including
one shared 16 MiB subtree behind the API routes that reaches every integration
client. A dashboard serves a handful of routes per session.

`experimental.preloadEntriesOnStart: false` is Next's documented switch for this.
An earlier round of this work recorded route preloading as "framework behaviour, not
something this codebase sets" - that was wrong, and the findings doc is corrected.

Deterministic per-stage measurements, identical across both runs on each side:

  stage            source loaded      files        heapUsed
  boot idle       24.0 -> 10.6 MiB   1114 -> 706   70.7 -> 44.8  (-37%)
  post-login      27.9 -> 19.3 MiB   1215 -> 884   123.8 -> 109.8
  board loaded    29.0 -> 21.7 MiB   1323 -> 1006  (run-to-run noise)
  multi-tab       29.2 -> 22.9 MiB   1325 -> 1015  139.7 -> 127.8
  post-soak       29.2 -> 22.9 MiB   1325 -> 1015  142.5 -> 127.4

The important part is the last two rows: 310 files and 6.3 MiB of source stay
unloaded even after a full multi-tab session, so this is not merely deferred cost -
routes the session never visits are never paid for at all. Live heap is lower at
every stage, by 26 MiB at idle and 12-15 MiB under load and at rest.

Peak node RSS does not move (350.3 -> 346.1, spreads overlap). V8 sizes its heap
against available headroom, so a smaller live set does not shrink peak RSS; what
improves is idle footprint and how much room is left before the heap cap. Median
board load is +3% with overlapping spreads, so the first-hit cost this trades for
did not show at this workload.

Full E2E suite passes 8/8 against the built image.

apps/nextjs/next.config.ts

@@ -87,6 +87,17 @@ const nextConfig: NextConfig = {
     optimizePackageImports: ["@mantine/core", "@mantine/hooks", "@tabler/icons-react"],
     turbopackFileSystemCacheForBuild: true,
     useTypeScriptCli: true,
+    /**
+     * Next otherwise loads the module graph for every route at startup. Measured here: 74 route
+     * bundles and 1,114 files, 24 MiB of source, resident before the first request — and one
+     * shared 16 MiB subtree behind the API routes that reaches every integration. A dashboard
+     * serves a handful of routes per session, so preloading the rest is pure cost.
+     *
+     * The trade is that the first request to a route pays its module load instead of the boot
+     * paying for all of them. That suits a long-lived self-hosted server, where boot happens
+     * once and routes warm up as they are used.
+     */
+    preloadEntriesOnStart: false,
   },
   turbopack: {
     root: path.resolve(import.meta.dirname, "../.."),