Docs/onboarding env webhooks

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Closes #21

[ogazboiz]

solid docs pass. node version lines up with the dockerfile (node:22-alpine), every required env var in env.ts is now in both the readme table and .env.example, and the schedulers plus the x-remitlend-signature hmac flow are clearly documented. nice touch extracting createWebhookSignature/verifyWebhookSignature with a timing-safe compare, and the deletions only trim aspirational placeholder sections, nothing load-bearing. build, typecheck and the full test suite all pass.

one thing before merge: npx prettier --check . fails on README.md (the markdown tables need prettier's column padding) and it was clean on main, so run npm run format and recommit. once that is green this is good to go.

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@ogazboiz merge my code