Feat/add pvlive nighttime zeros

[AdityaShome]

Description

This PR implements nighttime zeros for PVLive data when no generation data is available.

Changes:

• Added nighttime.py module with make_night_time_zeros() function that uses pvlib to calculate solar elevation
• When solar elevation < 5°, synthetic zero-generation rows are created for 30-minute intervals
• Added uk_gsp_locations.csv with latitude/longitude data for 333 GSPs
• Integrated into fetch_gb_data_historic() - only triggers when PVLive returns empty results
• Added pvlib dependency to pyproject.toml
• Includes 5 comprehensive unit tests

Approach:
This implementation addresses all feedback from PR #110:

• Uses local CSV for GSP locations (no external dependency on nowcasting_datamodel)
• Minimal changes to existing code, only adds zeros when PVLive returns no data
• Uses pvlib solar elevation calculation (same as pvlive-consumer)
• Only active for regime="in-day"

Fixes #104

How Has This Been Tested?

Unit Tests:

• Winter midnight returns zeros (nighttime)
• Summer midday returns empty (daytime)
• Non "in-day" regime returns empty
• Invalid GSP ID handled gracefully
• DataFrame schema validation (correct columns, 30min intervals)

All 5 tests pass:

uv run pytest tests/unit/test_nighttime.py -v
# all 5 passed 

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