speed up gsp query

[peterdudfield]

Detailed Description

One of the slowest database queries is getting the latest gsp capacites. This should be really quick, as its only 318 rows of data

We should speed this up

Context

• good to keep things speed up
• by using a start datetime filter in the sql filter, the query will speed up

Possible Implementation

1. add datetime_utc to here and pass through to here, this input should be optional
2. add datetime_utc = now-2 days in [here](https://github.com/openclimatefix/pvnet_app/blob/main/pvnet_app/app.py#L283C26-L283C51_ in the pvnet_app