feat(demo): bring up coordinator + userservice, seed root admin

make demo-ui now starts the userservice (own Postgres + migrations) beside the
coordinator on a shared JWT secret, and seeds a root admin. The coordinator runs
in UI session mode, so /ui opens a login page instead of a basic-auth prompt.

- docker-compose.users.yml overlay: userservice stack + coordinator JWT wiring
- demo-ui.sh: waits for the userservice, logs in as the seeded admin to poll the
  now session-gated dashboard, and prints the admin credentials
- validated with docker compose config (6 services, merged env)

coordinator/Makefile

@@ -35,7 +35,7 @@ help:
 	  '  make demo-down               Stop the demo services and workers.' \
 	  '  make test / make vet         Run Go verification.' \
 	  '' \
-	  'Demo UI: http://localhost:18080/ui (operator / demo-ui-secret).'
+	  'Demo UI: http://localhost:18080/ui (login page; admin root@scimesh.local / rootpassword).'
 
 demo-ui:
 	@DEMO_PROJECT="$(DEMO_PROJECT)" \