udp/config: fall back to default_owner when os_info has no owner; log debug

- When os_info arrives with no owner field, apply default_owner from server config
- Stop applying default_owner unconditionally in get_host_access (now deferred to os_info handling)
- os_info plugin logs debug message when injecting owner from client config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/config.py

@@ -309,7 +309,7 @@ def get_host_access(config, hostname) -> dict:
     """
     host_cfg = get_host_config(config, hostname)
 
-    owner = host_cfg.get("owner") or get_default_owner(config)
+    owner = host_cfg.get("owner") # or get_default_owner(config)
 
     managers = host_cfg.get("managers", [])
     if isinstance(managers, str):