refactor: merge vendor user into saas-admin

The admin user IS the platform admin — no separate vendor user needed.
The saas-vendor role is now always assigned to the admin user during
bootstrap. Removes VENDOR_ENABLED, VENDOR_USER, VENDOR_PASS from all
config, prompts, compose templates, and bootstrap script.

In multi-tenant mode: admin logs in with saas-admin credentials, gets
platform:admin scope via saas-vendor role, manages tenants directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -77,9 +77,6 @@ services:
       PG_DB_SAAS: ${POSTGRES_DB:-cameleer_saas}
       SAAS_ADMIN_USER: ${SAAS_ADMIN_USER:-admin}
       SAAS_ADMIN_PASS: ${SAAS_ADMIN_PASS:-admin}
-      VENDOR_SEED_ENABLED: "${VENDOR_SEED_ENABLED:-false}"
-      VENDOR_USER: ${VENDOR_USER:-vendor}
-      VENDOR_PASS: ${VENDOR_PASS:-vendor}
     healthcheck:
       test: ["CMD-SHELL", "node -e \"require('http').get('http://localhost:3001/oidc/.well-known/openid-configuration', r => process.exit(r.statusCode === 200 ? 0 : 1)).on('error', () => process.exit(1))\" && test -f /data/logto-bootstrap.json"]
       interval: 10s