feat: add Stripe webhook support

[VeldtJumper]

Summary

• add first-class Stripe webhook support for classic snapshot events and Stripe v2 thin events
• format event-aware amounts, IDs, statuses, changed fields, timestamps, and currencies into bounded Discord embeds
• register Stripe with a canonical example, website tile/logo, and provider documentation
• cover documented bracketed v2 event types, malformed payloads, privacy boundaries, Discord limits, API registration, and packaged examples

Verification

• npm test — 221 passed
• npm run lint
• npx tsc --noEmit
• npm run build
• cd web && npm test — 1 passed
• cd web && npm run build
• Docker build/runtime smoke test for provider registration, ready route, compiled provider, and packaged example