Trial Confirm Card

T2-6 (BILLING_RULES.md §13): finalize card-add for a trial creator.

Frontend calls this after Stripe Elements confirms a SetupIntent. We:

1. Verify the user is in trial_card_pending state
2. Verify a payment method is actually attached to their Stripe customer
3. Set users.card_added_at = NOW(), trial_status = 'active'
4. Mark the advisory invoice (if any) as status='waived'
5. Return the new state to the caller

Real billing begins on the NEXT 26th-of-M+1 cycle. The past advisory cycle is waived (free) per §13 rule "Card added within 5 days".

Idempotent: subsequent calls when state is already 'active' are no-ops that return success with already_active=True.