[Feature]: Last Successful refresh in feed API

Feature Summary

Store and return the last time a feed was successfully refreshed

What problem does this feature solve?

Allow API users to prioritize feeds that were not successfully refreshed recently.

I use the APIs to slowly refresh some feeds on the same domain so I don't overload that domain. Even with this careful approach, I occasionally hit rate limits on that domain. I use the Feed API https://miniflux.app/docs/api.html#endpoint-get-feed to start with the feed that was refreshed longest ago. But the Last Refreshed parameter only tells me when the last attempted refresh was, not when the last successful refresh was. It would be better to start with the feed that was successfully refreshed furthest in the past, but that would require an API parameter that stores the last successful refresh.

Proposed Solution

Add a Last Successful Refresh parameter to the Feed API.

Alternatives Considered

Scrape the page for failed refreshes.

Additional Context

No response

Checklist

• I have searched existing issues to ensure this feature hasn't been requested before.
• I understand that feature requests are not guaranteed to be implemented.
• I agree to follow the project's contribution guidelines.