feat: add deployment mode — vendor (multi-tenant) or single-tenant

Installer now asks deployment mode in simple mode:
- Multi-tenant vendor: creates saas-vendor role + assigns to admin
- Single tenant: asks for org name, creates Logto org + tenant record,
  assigns admin as org owner

Reverts always-create-vendor-role — role is only created when vendor
mode is selected. TENANT_ORG_NAME env var passed to bootstrap for
single-tenant org creation.

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

docker-compose.yml

@@ -80,6 +80,7 @@ services:
       VENDOR_SEED_ENABLED: "${VENDOR_SEED_ENABLED:-false}"
       VENDOR_USER: ${VENDOR_USER:-vendor}
       VENDOR_PASS: ${VENDOR_PASS:-vendor}
+      TENANT_ORG_NAME: ${TENANT_ORG_NAME:-}
     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