Align UDM WAN Monitor with the module secret contract

[itsDNNS]

Summary

• declare udm_wan_password through the scoped module secret contract
• add explicit saved-secret metadata to the UDM WAN Monitor Settings template
• bump the module and registry entry to 3.1.3
• validate catalog manifests against the authoritative core contract in CI
• document string defaults, ownership boundaries, masking, and preserve-on-save behavior

Compatibility

Requires the scoped secret contract merged in itsDNNS/docsight#753.

Testing

• registry validation passed
• 6 catalog unit tests passed
• all 4 catalog manifests passed the merged core contract validator
• git diff --check passed

Closes itsDNNS/docsight#752