Add budget-window batch economy endpoint

[MaxGhenis]

Summary

• add a dedicated budget-window economy endpoint that batches yearly reform-impact work server-side
• cap active yearly jobs and return aggregate progress, completed years, queued years, and final totals
• cover completed, queued, and error flows in the economy service tests

Testing

• FLASK_DEBUG=1 uv run --python 3.12 pytest tests/unit/services/test_economy_service.py
• uv run --python 3.12 ruff format --check .

Companion app change: PolicyEngine/policyengine-app-v2#930

[codecov]

Codecov Report

❌ Patch coverage is 76.21359% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.85%. Comparing base (6af28d7) to head (cdfbf01).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/services/economy_service.py	74.07%	24 Missing and 18 partials ⚠️
policyengine_api/routes/economy_routes.py	85.36%	3 Missing and 3 partials ⚠️
policyengine_api/country.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3387      +/-   ##
==========================================
+ Coverage   72.20%   72.85%   +0.64%     
==========================================
  Files          56       56              
  Lines        2425     2656     +231     
  Branches      423      476      +53     
==========================================
+ Hits         1751     1935     +184     
- Misses        595      622      +27     
- Partials       79       99      +20     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.