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/conftest.py

@@ -0,0 +1,22 @@
+"""Pytest configuration and shared fixtures.
+
+Disposes the global DB engine after each test to avoid ResourceWarning
+from unclosed sqlite3 connections when tests touch duty_teller.db.session.
+"""
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _dispose_db_engine_after_test():
+    """After each test, dispose the global engine so connections are closed."""
+    yield
+    try:
+        import duty_teller.db.session as session_mod
+
+        if session_mod._engine is not None:
+            session_mod._engine.dispose()
+            session_mod._engine = None
+            session_mod._SessionLocal = None
+    except Exception:
+        pass