PocketBase had no CI build path: `ghcr.io/copilotkit/showcase-pocketbase`
was a stale April `:latest`, and there was no way to ship pb_migrations /
pb_hooks changes without an ad-hoc manual build. Add a `pocketbase` slot to
showcase_build.yml's build matrix, mirroring the harness/aimock entries:
- dispatch_name `showcase-pocketbase`, context `showcase/pocketbase`, its
own Dockerfile, health `/api/health`, railway_id from the SSOT.
- a paths-filter key gated to `showcase/pocketbase/**` so the slot only
rebuilds when PB's own files change (the image is self-contained — no
shared-module copy), not on every showcase push.
- the workflow_dispatch service choice so PB is human-targetable.
Flip the SSOT entry (railway-envs.ts) to `ciBuilt: true` with
`dispatchName: "showcase-pocketbase"` so it is built+pushed (`:sha` +
`:latest`) and joins the default staging-redeploy scope; the build's
redeploy step only touches the matrix-intersect-success set, so PB still
only redeploys when its own files change. Regenerate
railway-envs.generated.json and the showcase_promote.yml service dropdown,
and update the SSOT/redeploy tests that pinned PB as out-of-band
(CI_BUILT_SERVICES 25 -> 26; webhooks stays the only non-CI-built service).