Add support for poll intervals smaller than 15 minutes (foreground service)

I regularly need to get notified of certain events in the shortest possible amount of time. Due to regional restrictions and technical limitations, SMS notification and push notifications (eg. via Pushr) are not available to me. So email is the only available option to me.

Before the most recent update (the one with the UI change) I was able to set the fetch interval of individual mail accounts to very low settings. I always did that temporarily and with the explicit permission of the mailservers owner and administrator. (By the way: "Email push" is not available to me and has proven to be unreliable in the past, when I was on a different setup.)

Unfortunately with the new update, the shortest configurable fetch interwal is 15 minutes. I do understand that this is fine for most use cases but it puts me in a difficult situation. 15 minutes are at least 10 minutes too long (if not more) if I have to get notified asap.

Describe the solution you'd like
I do understand that I might be somewhat of an edge case but I do want to politely ask for allowing users to set fetch intervals blow 15 minutes.

Describe alternatives you've considered
If the removal of short fetch intervals was a measure to protect people from themselves, I want to suggest to hide the option for short fetch intervals inside a debug menu or "advanced" menu, which would prevent casual users from accidentally choosing "too short" fetch intervals.

Start by locating the fetch-interval setting introduced or changed in the recent UI update, then trace how the foreground service performs scheduled account fetches. Check how the current 15-minute minimum is enforced. Done means users can configure intervals below 15 minutes, with the behavior and any advanced-setting restriction covered by the relevant tests.