fix(ci): add --check-untyped-defs to mypy and fix test typing

[paruff]

Summary

Adds --check-untyped-defs to the pre-commit mypy hook so untyped functions (mostly test helpers) are now type-checked too, and fixes all 97 errors this surfaced.

What changed

• .pre-commit-config.yaml: mypy args now include --check-untyped-defs
• core/integrations/market/census.py: widen discover_places_in_state return to dict[str, Any]
• core/services/portfolio.py: widen monthly_income_series/compute_portfolio_performance returns to dict[str, Any]
• core/management/commands/import_csv.py, populate_growth_areas.py: explicit typing for CSV row values / census values
• 19 test files: narrowed dict | None results, replaced resp.url with resp["Location"], annotated mock datasets, narrowed Django Field unions, fixed timezone.timedelta → datetime.timedelta

Validation

• pre-commit all green (ruff, mypy 296 files, djLint, gitleaks, etc.)
• python manage.py test: 99 + 23 + 38 tests OK
• pytest core/tests: 115 passed