[DEPRECATED] Add XML config support to user-service

Context

Part of XML configuration migration initiative across all services.

Implementation

• XML parser (JAXB)
• Config schema definition
• Backward compatibility with JSON
• Auto-migration tool

Dependencies

• Requires streaming-service XML migration first
• Shared XML schema library

Timeline

• Started: Oct 1, 2025
• 3 days already invested
• Est. 4 more days to complete

UPDATE: Architecture team announced YAML as new standard. XML work deprecated.