Verifies the shared-secret Authorization header, then maps
RevenueCat's app_user_id (the restaurant id — mobile must configure
Purchases with appUserID: restaurantId) to an upsert into
subscriptions. INITIAL_PURCHASE/RENEWAL/PRODUCT_CHANGE/UNCANCELLATION
-> active, BILLING_ISSUE -> grace_period, EXPIRATION -> suspended.
CANCELLATION alone does not suspend — it only means auto-renew is
off, access continues until the period actually expires.
Tested end-to-end against the real Supabase project (create
restaurant, POST the webhook, verify the subscriptions row).