Add Hephaestus/caddy/Caddyfile - Caddy reverse-proxies both containers on
{$SITE_HOSTNAME} (from docker-compose .env): /auth/* -> auth-proxy:4000 with
the prefix stripped, / -> forgejo:3000 with a 2GB body cap for git push, LFS,
and release uploads. HTTPS via automatic Lets Encrypt (HTTP-01 on :80); SSH
git access stays on 2222 outside Caddy.
docker-compose.yml wires the new caddy service alongside the two existing
containers; auth-proxy/src/index.ts gets the minor upstream adjustment so
paths line up after Caddy strips the /auth prefix.
