test(integration): boot freight API in-process across parallel shards

The suite drove a containerized freight API, so every code change needed an
image rebuild before a test could see it, and there was no way to attach a
debugger. Files also ran strictly in sequence against one shared database,
which is the root of the warm-stack gotchas the README documents: stowaway
paid bookings climbing back aboard, a short consist on the fifth file.

The freight app now boots inside each vitest worker from dist/, and each
worker owns a whole shard of the topology - its own database, payment API,
gateway mock and broker vhost - so nothing mutable is shared and files run
in parallel. Full suite drops from roughly 20 minutes to 196s at 4 shards.

- main.ts exports createFreightApp() so the harness applies the same prefix,
  pipes, filters and interceptors as production instead of replaying them by
  hand; self-start is guarded by require.main so the Dockerfile CMD still boots
- booking-window tick cadence is env-driven (BOOKING_WINDOW_TICK_CRON), */1 in
  the suite, */10 unchanged in production
- prepare-shards.mjs seeds a template database (boot seeders, then the SQL
  fixtures that depend on them) and clones it per shard; it.mjs re-clones on
  every run, so each run is hermetic
- gateway mock and payment API are generated per shard: the mock keeps modes
  and orders process-global and 20 of 25 specs reset it in beforeAll, and the
  inbound CBE bill query has to reach one specific shard's app
- poll() samples every 250ms instead of 2000ms, keeping the caller's deadline
- authz.it.ts seeds its own invoice; it previously read another spec's leftover
  and returned early, which silently passed on a pristine database

Known: an unlocked MAX(sequence_no)+1 in train-scheduling.service.ts races
under concurrent allocation and leaves a short consist, so 1-3 specs fail
intermittently. Pre-existing and reproduces at the production tick cadence.

.gitignore

@@ -49,3 +49,6 @@ test-results/
 playwright-report/
 blob-report/
 RUNNING_LOCALLY.md
+
+# Generated per-shard compose file for the integration suite (it.mjs).
+integration/.it-shards.yaml