List external knowledge panel sources (JSON)

Returns the ordered list of external knowledge panel sources configured on the server.

Providers can use the knowledge_panel_url field to point to their own API. The URL may contain template variables that the frontend will expand before calling:

• $code → product barcode
• $lc → UI language (2 letters)
• $cc → user/device country (2 letters)

Example: https://provider.example.com/off/v1/knowledge-panel/$code?lang=$lc&country=$cc

The frontend percent-encodes the variable values when substituting them into the URL.