chore: update development dependencies and improve test coverage

- Upgraded `pytest-asyncio` to version 1.0 to ensure compatibility with the latest features and improvements.
- Increased the coverage threshold in pytest configuration to 80%, enhancing the quality assurance process.
- Added a new `conftest.py` file to manage shared fixtures and improve test organization.
- Introduced multiple new test files to cover various components, ensuring comprehensive test coverage across the application.
- Updated the `.coverage` file to reflect the latest coverage metrics.

tests/test_import_duty_schedule_integration.py

@@ -20,17 +20,23 @@ def db_url():
     return "sqlite:///:memory:"
 
 
+def _dispose_global_engine(session_module):
+    """Dispose global engine so connections are closed, then clear cache."""
+    if session_module._engine is not None:
+        session_module._engine.dispose()
+    session_module._engine = None
+    session_module._SessionLocal = None
+
+
 @pytest.fixture(autouse=True)
 def _reset_db_session(db_url):
     """Ensure each test uses a fresh engine for :memory: (clear global cache for test URL)."""
     import duty_teller.db.session as session_module
 
-    session_module._engine = None
-    session_module._SessionLocal = None
+    _dispose_global_engine(session_module)
     init_db(db_url)
     yield
-    session_module._engine = None
-    session_module._SessionLocal = None
+    _dispose_global_engine(session_module)
 
 
 def test_import_creates_users_and_duties(db_url):