The caddy sidecar uses on-demand TLS and asks http://app:3000/tls-check before issuing a cert.
On a shared host every co-located stack aliases its main service 'app' on the 'proxy' overlay;
caddy (on both proxy+internal) resolves bare 'app' to a FOREIGN stack's endpoint, so the tls-check
connection is refused, no cert is issued, and the PDS is unreachable over HTTPS (xrpc/_health=000).
Give the PDS a unique 'pds' alias on the internal network and point caddy's reverse_proxy +
on_demand_tls ask at it; 'pds' exists only on internal, so it always resolves to this stack's PDS.
Service name stays 'app' (no downstream breakage).
