Teslamate: Allow negative charge costs

[russinating]

Many Tesla owners in the UK are with Octopus Energy, a provider who's half-hourly rates are based on wholesale costs. Occassionally this can result in negative prices, ie being paid to use electricity.

Currently Teslamate prevents setting charge costs as negative, requiring them to be 0 or greater. If this is simply human validation and not a larger job, affecting the database and calculations, can this be amended to allow for negative values please?

[adriankumpf]

Somewhat related to #655 (Octopus Energy)

[cwanja]

_Paid_ to use electricity?! What am I doing wrong.

[russinating]

_Paid_ to use electricity?! What am I doing wrong.

I know! Lockdown has helped. Lowest rate tomorrow is -11.81p/kW @ 14:30-15:00 and along with the rest of the afternoon I should be paid ~£4 for charging the M3. Small gains!

[russinating]

@adriankumpf

I manually updated my charge cost today to a negative via psql and it appears to be displaying correctly and doesn't seem to have broken any grafana calcs so I presume the prevention is/was just for human error.

By the way, thank you for all your hard work on this, I've been meaning to donate since I started using it a few weeks ago but got around to it this evening.