chore: add coverage reporting and improve documentation

- Added `pytest-cov` as a development dependency for coverage reporting.
- Configured pytest to include coverage options, ensuring code coverage is reported and enforced.
- Updated the README to include contributing guidelines and logging policies, enhancing clarity for developers.
- Added a new section in the configuration documentation emphasizing the necessity of serving the application over HTTPS in production for security purposes.
- Introduced a new `.coverage` file to track test coverage metrics.

duty_teller/services/import_service.py

@@ -21,6 +21,7 @@ def _consecutive_date_ranges(dates: list[date]) -> list[tuple[date, date]]:
     ranges: list[tuple[date, date]] = []
     start_d = end_d = sorted_dates[0]
     for d in sorted_dates[1:]:
+        # Merge consecutive days into one range; gap starts a new range
         if (d - end_d).days == 1:
             end_d = d
         else: