Octopus Home Mini live meter reading needs caching

[ryanw-mobile]

This is the real issue:

This determines how often data related to your Home Mini is retrieved from Octopus Energy. The Octopus Energy APIs have a rate limit of 100 calls per hour, which is shared among all calls including through the app.

Our latest implementation to get a reading every minute takes up to 60 API calls if we do not navigate to other screens.
Since we don't cache API results if we hop between screens AND trigger other API calls (like getting usage) it is very likely we hit the limit very quickly.

Since we no longer get 10-seconds readings, it brings more value if we cache the latest 1-minute live meter reading.