Rewrite the service in Go and ship it as a static container

The PHP app, Apache, and .htaccess are replaced by a single Go binary
(standard library only). Routing, the JSON response, the landing page,
the shared email design, and all four parsers are ported with the same
output; JSON object keys are kept in sender order via an ordered decoder.

- mail.go: SMTP delivery with quoted-printable body and RFC 2047 subject
- main.go: routing, client IP from the proxy, /healthz, -check flag
- render.go: email shell, escaping, table helpers, generic renderer
- parser_*.go: github, grafana, json, wxinteract
- main_test.go: handler, response shape, IP forwarding, every parser
- Dockerfile: multi-stage build to distroless static, non-root, HEALTHCHECK
- deploy/webhooks.dixon.cx.service: systemd unit for the container
- README and PARSER_PROMPT updated for Go; IMPLEMENTATION_SUMMARY removed

WEBEX_INTERACT_GUIDE.md

@@ -336,7 +336,7 @@ If you're receiving raw webhook data instead of formatted emails:
 
 1. **Check URL:** Ensure `/wxinteract` is at the end of the URL
 2. **Check payload structure:** Parser expects `{"data": {"status": "...", ...}}`
-3. **Check PHP logs:** Look for any parser errors in your server logs
+3. **Check the container logs:** `docker logs webhooks.dixon.cx` shows mail failures and parser errors
 
 ### Missing Fields