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

Dockerfile

@@ -0,0 +1,12 @@
+FROM golang:1.26-alpine AS build
+WORKDIR /src
+COPY go.mod ./
+COPY *.go docs.html ./
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /webhooks .
+
+FROM gcr.io/distroless/static-debian12:nonroot
+COPY --from=build /webhooks /webhooks
+ENV LISTEN=:8080 SMTP_ADDR=172.17.0.1:25 MAIL_FROM=no-reply@dixon.cx PUBLIC_HOST=webhooks.dixon.cx
+EXPOSE 8080
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s CMD ["/webhooks", "-check"]
+ENTRYPOINT ["/webhooks"]