fix(payments): read the clover field paths in the platform Stripe webhook (#544)

lib/stripe.ts pins apiVersion 2026-02-25.clover, where
Subscription.current_period_start/end moved onto SubscriptionItem and
Invoice.subscription moved to parent.subscription_details.subscription.
The student webhook was migrated for this; the platform webhook was not,
and it reads through an `any` cast so TypeScript never flagged it.

Every handler was therefore dead in production:

- checkout.session.completed and customer.subscription.updated did
  `new Date(undefined * 1000).toISOString()` → RangeError, thrown while
  building the upsert object, before any write. A school paid and got no
  platform_subscriptions row, no tenants.plan, no revenue split; the
  route 500'd and Stripe retried forever.
- invoice.payment_failed / invoice.paid resolved an undefined
  subscription id and silently broke out, so past_due was never set and
  the dunning email never sent.

Mirrors the accessors in lib/payments/stripe-provider.ts:454-460, with
the pre-clover paths kept as `??` fallbacks. toIso() guards the numeric
check so a future shape change degrades (period column omitted, error
logged) instead of 500-ing.

Also in the same route:

- Idempotency: events are recorded in webhook_events under provider
  'stripe_platform' before handling and replayed ids are a no-op, same
  as app/api/payments/webhook/[provider].
- Ordering: customer.subscription.updated compares the event's period
  end against the stored one and drops strictly-older events whole,
  so an out-of-order delivery cannot rewind current_period_end /
  tenants.billing_period_end (which feed the expiry cron and the "next
  payment" date) or the plan via applyPortalPlanChange.
- Errors: every read/write goes through unwrap() and a failure now 500s
  so Stripe retries, instead of being discarded behind a 200.
- Stripe statuses outside the platform_subscriptions CHECK set (today
  'paused') map to past_due instead of failing the constraint; the
  checkout metadata interval is pinned to monthly|yearly the same way.

tests/unit/platform-webhook.test.ts drives POST with clover-shaped
fixtures for all five handled event types. 18 of its 28 cases fail
against the pre-fix route.

Closes #544

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RnvphWuXtdxC2fkRCcw3t9

Author: guillermoscript