SetupTracing merged resource.Default() (schema 1.43.0 under otel 1.45.0)
with semconv/v1.24.0's schema URL, so resource.Merge errored with
'conflicting Schema URL' and the backend exited at startup whenever an
OTLP endpoint was configured. resource.NewSchemaless keeps service.name
without pinning a schema, surviving otel major bumps.
Shared NAT egress makes client IP unusable for uniqueness checks. Gate the
behavior behind REGISTRATION_ENFORCE_IP (default false) for public/invite signup.