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

deploy/webhooks.dixon.cx.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=webhooks.dixon.cx (docker container)
+After=docker.service network-online.target
+Requires=docker.service
+Wants=network-online.target
+
+[Service]
+Restart=always
+RestartSec=5
+TimeoutStartSec=0
+ExecStartPre=-/usr/bin/docker rm -f webhooks.dixon.cx
+ExecStart=/usr/bin/docker run --rm --memory 256m --cpus 1.0 --pids-limit 128 --name webhooks.dixon.cx -p 127.0.0.1:29593:8080 webhooks.dixon.cx:latest
+ExecStop=/usr/bin/docker stop -t 30 webhooks.dixon.cx
+
+[Install]
+WantedBy=multi-user.target