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_service.py

@@ -0,0 +1,47 @@
+"""Tests for duty_teller.services.import_service (_consecutive_date_ranges)."""
+
+from datetime import date
+
+import pytest
+
+from duty_teller.services.import_service import _consecutive_date_ranges
+
+
+class TestConsecutiveDateRanges:
+    """Unit tests for _consecutive_date_ranges."""
+
+    def test_empty_returns_empty(self):
+        assert _consecutive_date_ranges([]) == []
+
+    def test_single_day_one_range(self):
+        d = date(2025, 2, 10)
+        assert _consecutive_date_ranges([d]) == [(d, d)]
+
+    def test_consecutive_days_one_range(self):
+        dates = [date(2025, 1, 10), date(2025, 1, 11), date(2025, 1, 12)]
+        assert _consecutive_date_ranges(dates) == [
+            (date(2025, 1, 10), date(2025, 1, 12)),
+        ]
+
+    def test_two_ranges_with_gap(self):
+        dates = [
+            date(2025, 1, 5),
+            date(2025, 1, 6),
+            date(2025, 1, 10),
+            date(2025, 1, 11),
+        ]
+        assert _consecutive_date_ranges(dates) == [
+            (date(2025, 1, 5), date(2025, 1, 6)),
+            (date(2025, 1, 10), date(2025, 1, 11)),
+        ]
+
+    def test_unsorted_with_duplicates_normalized(self):
+        dates = [
+            date(2025, 1, 12),
+            date(2025, 1, 10),
+            date(2025, 1, 11),
+            date(2025, 1, 10),
+        ]
+        assert _consecutive_date_ranges(dates) == [
+            (date(2025, 1, 10), date(2025, 1, 12)),
+        ]