feat: add RevenueCat webhook support

[VeldtJumper]

Summary

• add a RevenueCat webhook provider for subscription lifecycle, transfer, virtual currency, experiment, purchase-redemption, Paywall UI, custom, and future event types
• register /revenuecat with a canonical official example and defensive Discord-safe formatting
• add RevenueCat to the README and marketing site with a pinned, attributed provider icon

Verification

• npm test — 232 tests passed
• npm run lint
• npm run build
• cd web && npm test && npm run build
• production Docker image build and packaged-example/container HTTP smoke checks
• local API/site smoke checks and headless visual inspection of the provider grid