Add tests for category budgets and new functionality

New test files added:
- test_category_edge_cases.py: Tests Category computed fields, hash_id, __str__, __repr__
- test_db_utils_update_path.py: Tests for updated upsert() path handling
- test_models_common.py: Tests Sign enum, date2str, ExportBaseModel methods
- test_models_init.py: Tests for lazy loading from models.__init__.py

Code changes:
- Fixed CategoryBudget to have default=None for optional valid_from/valid_to fields
- Updated src/models/__init__.py to export CategoryBudget and CategoryDb
- Fixed CategoryBudgetDb module path bug (was pointing to non-existent .category_budget)
- Improved db_utils.upsert() to handle partial overlaps correctly

Coverage improvements:
- New functionality coverage: 100%
- Overall codebase coverage: 86% (up from 83%)

Notes:
- Some existing domain_utils.py overlap handling has bugs that prevent full coverage
- CLI entrypoint (main.py) and some enum methods remain uncovered but are optional

pyproject.toml

@@ -14,6 +14,7 @@ dependencies = [
     "mypy>=2.4.0",
     "pydantic>=2.13.5",
     "pytest>=9.1.1",
+    "pytest-cov>=7.1.0",
     "python-dotenv>=1.2.3",
     "sqlalchemy>=2.0.52",
 ]