…27.dahlia)

Sits at the bottom of the deploy-billing stack so every change above it is
written and tested against one SDK/API version. Full breaking-change audit in
ENG-2895; this change migrates the code that exists on main:

- API version pins -> 2026-05-27.dahlia (lib/stripe.ts, webhook route).
- New lib/stripe/invoiceCompat.ts: dual-shape readers for invoice
  subscription / payment intent / line price / line proration and the
  per-item subscription period (basil reshaped all of these). Webhook payload
  shapes follow the ENDPOINT's pinned API version, not the SDK's, so the
  helpers accept both shapes; after deploy, bump the webhook endpoint to
  2026-05-27.dahlia in Stripe, then the legacy halves can be removed.
- paymentUtils reads all go through the compat helpers.
- isAutomatedBillingRenewal compares only fields present in
  previous_attributes: basil+ endpoints report renewals as per-item
  current_period_* diffs, which the old comparison misread as plan changes.
- createSubscription pins billing_mode classic (clover defaults new
  subscriptions to flexible, which itemizes prorations differently).
- handleStripeError: v22 exports error classes as values, not types.

Stack changes above this one carry their own dahlia-isms: subscribeDeploy
billing_mode pin and price.product reads (deploy-subscribe-flows),
invoices.createPreview (deploy-project-gate), credit grants (ENG-2870).

ENG-2895