Fix double-escaped HTML entities in package READMEs

[dimitrieh]

Some package READMEs (e.g., node-red-opcua-x) were displaying HTML tags as literal text instead of rendering them. This was caused by HTML entities being stored in escaped form (<img> instead of ) in the database.

The fix unescapes HTML entities before markdown processing to handle cases where HTML tags are stored as <img> instead of . This ensures already-escaped content from storage renders properly while maintaining security through DOMPurify sanitization.

See https://flows.nodered.org/node/node-red-opcua-x

Fixes https://github.com/node-red/flow-library/issues/122