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_db_session.py

@@ -0,0 +1,39 @@
+"""Tests for duty_teller.db.session (session_scope rollback/close, get_engine)."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from duty_teller.db import session as session_mod
+
+
+def test_session_scope_rollback_on_exception():
+    """session_scope: on exception inside block, rollback and close are called."""
+    mock_session = MagicMock()
+    with patch.object(session_mod, "get_session", return_value=mock_session):
+        with pytest.raises(ValueError):
+            with session_mod.session_scope("sqlite:///:memory:") as s:
+                assert s is mock_session
+                raise ValueError("test")
+    mock_session.rollback.assert_called_once()
+    mock_session.close.assert_called_once()
+
+
+def test_session_scope_close_on_success():
+    """session_scope: on normal exit, close is called (no rollback)."""
+    mock_session = MagicMock()
+    with patch.object(session_mod, "get_session", return_value=mock_session):
+        with session_mod.session_scope("sqlite:///:memory:") as s:
+            assert s is mock_session
+    mock_session.rollback.assert_not_called()
+    mock_session.close.assert_called_once()
+
+
+def test_get_engine_same_url_returns_same_instance():
+    """get_engine: same URL returns the same engine instance."""
+    session_mod._engine = None
+    session_mod._SessionLocal = None
+    url = "sqlite:///:memory:"
+    e1 = session_mod.get_engine(url)
+    e2 = session_mod.get_engine(url)
+    assert e1 is e2