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_package_init.py

@@ -0,0 +1,16 @@
+"""Tests for duty_teller package __init__ (version fallback)."""
+
+import importlib
+from importlib.metadata import PackageNotFoundError
+from unittest.mock import patch
+
+import duty_teller
+
+
+def test_version_fallback_when_package_not_installed():
+    """When version('duty-teller') raises PackageNotFoundError, __version__ is '0.1.0'."""
+    with patch("importlib.metadata.version", side_effect=PackageNotFoundError):
+        importlib.reload(duty_teller)
+    assert duty_teller.__version__ == "0.1.0"
+    # Restore so other tests see normal version
+    importlib.reload(duty_teller)