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

src/models/category.py

@@ -83,9 +83,13 @@ class CategoryBudget(BaseModel):
     category: Category
     amount_eur: Decimal = Field(gt=0, decimal_places=2)
     valid_from: date | None = Field(
+        default=None,
         description="Start of validity. None means unbounded"
     )
-    valid_to: date | None = Field(description="End of validity. None means unbounded")
+    valid_to: date | None = Field(
+        default=None,
+        description="End of validity. None means unbounded"
+    )
 
     @model_validator(mode="after")
     def validate_date_range(self) -> Self: