test(billing): add Stripe E2E webhook lifecycle tests
16 tests covering the full Stripe webhook flow through /billing/webhook/stripe:
- Subscription creation (customer.subscription.created → DB row)
- Period end extraction from items (Stripe API 2026-02+ compatibility)
- Billing customer creation
- Status updates (active, past_due, trialing)
- Cancellation (customer.subscription.deleted → cancelled)
- Payment failure (invoice.payment_failed → past_due)
- One-time payments (checkout.session.completed mode=payment)
- Full lifecycle: create → update → recover → cancel
- Edge cases: missing metadata, empty items, invalid JSON, bad signatures
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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