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_utils.py

@@ -97,3 +97,8 @@ def test_parse_handover_invalid():
     assert parse_handover_time("not a time") is None
     # 25:00 is normalized to 1:00 by hour % 24; use non-matching string
     assert parse_handover_time("12") is None
+
+
+def test_parse_handover_invalid_timezone_returns_none():
+    """Invalid IANA timezone string -> ZoneInfo raises, returns None."""
+    assert parse_handover_time("09:00 NotAReal/Timezone") is None