Add server health check for missing PHP curl extension

[dgershman]

Summary

• Adds server_info (PHP version, curl availability/version) to the settings API response for admin users only
• Displays a warning notice on the settings page when the PHP curl extension is missing
• Displays a success notice with PHP and curl version info when curl is available
• Prevents hours of debugging slow page loads caused by WordPress falling back to PHP streams without curl

Context

We spent hours debugging why na-wt.org's events page took ~33s to load. The root cause was PHP 8.4-FPM missing the curl extension — WordPress fell back to PHP streams for outbound HTTP requests, adding ~5s per call. This was invisible to the admin.

Test plan

• With curl enabled: settings page shows green success notice with PHP/curl versions
• With curl disabled: settings page shows yellow warning with instructions
• Non-admin API requests to /settings do not include server_info
• composer test passes (514 tests)
• npm run build succeeds
• composer lint passes

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