Switch code formatter from Black to Ruff

[MaxGhenis]

Summary

• Replace Black with Ruff (ruff format) for code formatting
• Update Makefile format target to use ruff format .
• Replace Black GitHub Actions in CI with ruff format --check .
• Add ruff>=0.9.0 as a dev dependency in setup.py
• Reformat all Python files with ruff defaults (88-char line length)

Test plan

• CI lint job passes with ruff format --check .
• Verify make format works locally

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 69.62025% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.20%. Comparing base (f3a7d44) to head (c0e719e).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/endpoints/economy/compare.py	37.50%	9 Missing and 1 partial ⚠️
policyengine_api/endpoints/policy.py	0.00%	5 Missing ⚠️
policyengine_api/routes/report_output_routes.py	33.33%	2 Missing ⚠️
policyengine_api/services/report_output_service.py	33.33%	2 Missing ⚠️
policyengine_api/country.py	85.71%	1 Missing ⚠️
policyengine_api/services/household_service.py	66.66%	1 Missing ⚠️
policyengine_api/services/simulation_service.py	75.00%	1 Missing ⚠️
.../payload_validators/validate_set_policy_payload.py	0.00%	0 Missing and 1 partial ⚠️
policyengine_api/utils/singleton.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3317   +/-   ##
=======================================
  Coverage   72.20%   72.20%           
=======================================
  Files          56       56           
  Lines        2425     2425           
  Branches      423      423           
=======================================
  Hits         1751     1751           
  Misses        595      595           
  Partials       79       79           

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