SearXNG: Enable JSON format in settings.yml by default to support API integrations

[ckarnes]

Summary
The SearXNG LXC install script does not enable the json format in settings.yml by default. This prevents API integrations like OpenWebUI from working out of the box.

Current behavior
The default settings.yml does not include json in the search.formats list, so any tool querying SearXNG via its JSON API will fail silently.

Suggested change
Add json to the formats list in settings.yml during installation:

search:
  formats:
    - html
    - json

Why
SearXNG is commonly used as a search backend for AI tools like OpenWebUI. Without JSON format enabled, these integrations fail with no clear error message, making it difficult to diagnose. Enabling it by default would make the LXC immediately useful for this common use case without requiring manual configuration.