feat: mark proxy-originated errors and pin container DNS

- handle_errors sets X-Proxy-Error (dial|upstream) and X-Proxy-Error-Detail
  so clients with several proxies can fail over when the upstream was
  never reached, instead of treating the proxy's 502 as the upstream's
- explicit dns in compose: the container no longer inherits a host
  resolver that may disappear (systemd-resolved was switched off on the
  VM and every lookup failed until a restart)
- strip X-Proxy-Token from Caddy logs (it was logged in clear text)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-0.2.0
+0.3.0