VULN-001: Remove insecure fallback that allowed processing webhooks without
signature verification when STRIPE_WEBHOOK_SECRET was not set.
Changes:
- Add startup check that exits with error if STRIPE_WEBHOOK_SECRET is missing
- Remove JSON.parse fallback that bypassed signature verification
- Always use stripe.webhooks.constructEvent() for webhook validation
This prevents attackers from forging webhook events to manipulate
orders, subscriptions, or other payment-related data.
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