feat: add validation for generation values against site capacity

[anxkhn]

Pull Request

Description

• Implemented logic to compare uploaded PV generation values against the site's capacity.
• Raised HTTP 102 error if any generation value exceeds the site's capacity, with a custom error message informing the user.
• Added site capacity check in the /sites/{site_uuid}/pv_actual route.

Fixes #191

How Has This Been Tested?

• Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[anxkhn]

Hey @peterdudfield !
Here's my PR. This is my first time contributing to @openclimatefix
Can you help set it up properly locally and update the tests so it passes?