Post Signup Activate

Step 3 (confirm): verify the Stripe SetupIntent succeeded, create the Stripe Customer + trial Subscription, and atomically flip the User to status='active'. Wraps services/signup.py::activate_user_from_setup_intent (which already handles idempotency, the pending-only status guard, SetupIntent status check, and Stripe error rollback).

Replays with the same pending_id are idempotent — a second call after the row is already active returns the same userId without re-calling Stripe.

Lower rate-limit (10/minute vs 20/minute on setup-intent) since each activate call potentially creates Stripe resources.