feat(caddy): serve Audiobookshelf at a guarded abs.franksdank.cloud

Provider-first exposure: the IdP row (proxy provider abs-proxy + application
"Audiobookshelf Guard", claimed by the embedded outpost) already exists, so the
route can ship. The site block proxies everything to anubis:8923 like every other
guarded vhost, and the post-auth :8017 handler routes the host to
192.168.0.235:8000. The container carries ROUTER_BASE_PATH=/audiobookshelf, so
the app has no root: the bare / is redirected onto the base path inside the
post-auth handler, never pre-auth, so an anonymous client only ever meets the
gate.

dankclaw/pihole/addn-hosts

@@ -26,6 +26,7 @@
 192.168.0.143 romm.franksdank.cloud
 192.168.0.143 files.franksdank.cloud
 192.168.0.143 jelly.franksdank.cloud
+192.168.0.143 abs.franksdank.cloud
 192.168.0.143 radarr.franksdank.cloud
 192.168.0.143 lidarr.franksdank.cloud
 192.168.0.143 readarr.franksdank.cloud