Fix active category lookup and README usage examples

tests/test_domain_utils_budget_overlap.py

@@ -1,13 +1,13 @@
 """Comprehensive tests for set_category_budget overlap handling."""
 
-from datetime import date
+from datetime import UTC, date, datetime
 
 import pytest
 from sqlalchemy.orm import Session
 
 from src.domain_utils import find_category_db, set_category_budget
 from src.models import Category, CategoryBudget, CategoryBudgetDb
-from src.models.category import upsert_category
+from src.models.category import CategoryDb, upsert_category
 from tests.factories import CategoryFactory
 
 # NOTE: MYPY errors suppressed for test fixtures - see AGENTS.md for type checking notes.
@@ -615,6 +615,33 @@ class TestFindCategoryDb:
         result = find_category_db(db_session, Category(name="NonExistent"))
         assert result is None
 
+    def test_find_reintroduced_category_returns_active_version(
+        self, db_session: Session
+    ):
+        category = Category(name="Reintroduced")
+        historical = CategoryDb(
+            name=category.name,
+            valid_to=datetime(2026, 1, 1, tzinfo=UTC),
+            last_action="retired",
+        )
+        db_session.add(historical)
+        db_session.commit()
+
+        active = upsert_category(db_session, category, do_commit=False)
+        assert active is not None
+        db_session.commit()
+
+        result = find_category_db(db_session, category)
+        assert result is not None
+        assert result.id == active.id
+
+        set_category_budget(
+            db_session,
+            CategoryBudget(category=category, amount_eur=100.00),
+        )
+        budget = db_session.query(CategoryBudgetDb).one()
+        assert budget.category_id == active.id
+
 
 class TestSetCategoryBudgetErrorHandling:
     """Test error handling in set_category_budget."""