Add GET /api/onboarding/slug-available endpoint to check if a slug is
already taken. Frontend checks availability with 400ms debounce as the
user types and shows inline feedback. Submit button disabled when slug
is taken. POST /api/onboarding/tenant now returns 409 instead of 500
for duplicate slugs.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>