Added Party CSV Export Functionality

[notvasub]

This PR implements CSV export functionality that allows administrators to download party registrations for a specified date range, enabling officers to have physical copies of the information when digital access is unavailable.

Changes:

• Added export_parties_to_csv method to PartyService that accepts a list of Party models and generates a CSV file with the 11 columns highlighted in the TDD.
• Added graceful handling for missing relationships (null location, contacts, or accounts) by populating empty strings in the CSV.
• Added GET /api/parties/csv endpoint that accepts required start_date and end_date query parameters in YYYY-MM-DD format
• Implemented date validation with a 400 error response for invalid formats
• Added admin authentication requirement using existing authenticate_admin dependency
• Added a ton of router and service tests.

Closes #63

[github-actions]

Test Results Summary

281 tests  +21   281 ✅ +21   6s ⏱️ ±0s
  1 suites ± 0     0 💤 ± 0 
  1 files   ± 0     0 ❌ ± 0 

Results for commit 642468a. ± Comparison against base commit bfd9aa9.

♻️ This comment has been updated with latest results.

[manyuagashe]

All good except for the test issues I mentioned.

[naasanov]

LGTM. Thanks!