feat: deliver signed webhooks

[felixvippp-ai]

Closes #32

Summary

• Added real HTTP webhook delivery when recordEvent() emits matching events.
• Added wildcard * event matching.
• Generated a per-webhook whsec_... secret at registration and returned it only once.
• Hid webhook secrets from list and update responses while surfacing deadLetters.
• Added HMAC-SHA256 signatures in X-AgentPay-Signature plus delivery/event headers.
• Added bounded retry for transient failures and permanent dead-letter counting.
• Updated POST /api/v1/webhooks/:id/test to perform a real signed delivery.
• Added SSRF protection that rejects private, loopback, and link-local targets unless ALLOW_PRIVATE_WEBHOOKS=true.
• Added docs/webhooks.md documenting payloads, headers, signatures, retry behavior, dead letters, and SSRF controls.

Validation

• npm.cmd run build
• $env:NODE_ENV='test'; node --test dist/webhooks/deliver.test.js
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js
• npm.cmd run lint
• npx.cmd prettier --check src/events.ts src/routes/webhooks.ts src/store/state.ts src/routes/operational.test.ts src/webhooks/deliver.ts src/webhooks/deliver.test.ts docs/webhooks.md
• git diff --check

Full post-clean rebuild test result:

tests 74
suites 6
pass 74
fail 0
cancelled 0
skipped 0
todo 0

Note: the repository npm test script uses POSIX NODE_ENV=test syntax, which Windows cmd.exe cannot execute directly. I ran the same built test set with PowerShell's $env:NODE_ENV='test' before invoking node --test.

Security Notes

• Webhook targets are restricted to http and https.
• Private, loopback, and link-local addresses are rejected by default, including DNS names that resolve to private addresses.
• ALLOW_PRIVATE_WEBHOOKS=true is intended only for controlled local/test environments.
• The signature covers the exact JSON request body sent to subscribers.
• verifyWebhookSignature() uses constant-time comparison for receiver-side checks.
• Webhook secrets are not returned after creation and are not included in list/update responses.

[mikewheeleer]

thanks for the effort @felixvippp-ai 🙏 this issue isn't assigned to you — please claim an open unassigned issue first, then open your PR. closing for now, hope to see it back 🙌