test: run the suite in CI and cover the 0.5.0 fixes - #280

The repository had pytest, coverage and Ruff configured in pyproject.toml
but nothing that ran them, no pytest dependency, and a root `test.py` whose
entire body was `print("fake test")`. The one real test, added with #269,
has never been executed anywhere.

Add pytest to the dev group, narrow `testpaths` to `tests`, drop the
placeholder, and run the suite on every pull request and on pushes to master
across Python 3.11 to 3.14. `uv sync --locked` doubles as the lockfile check
the pull request workflows otherwise lack.

Ruff is deliberately not gated. It currently reports 431 findings on the
existing code, so a blocking job would fail on arrival. Cleaning that up is
its own piece of work.

The tests themselves cover the bugs fixed for 0.5.0, each written so that it
fails again if the fix is reverted:

  test_utils_datetime    both API timestamp shapes, offsets other than Z,
                         rejection of ambiguous values, UTC normalization
                         of the --start-date/--end-date option type
  test_models_dates      the date filter across both shapes and naive and
                         aware bounds, items carrying no usable date at all,
                         is_published including the AudioPart parent
                         fallback
  test_download_queue    the deadlock: failing jobs must not strand the
                         queue, the abort must stop queued work while
                         letting running downloads finish, and a run that
                         saw failures must raise
  test_download_cli      the command itself has to raise on a failed job;
                         testing raise_for_errors() alone would stay green if
                         the call were dropped from the command
  test_exceptions        ItemNotPublished must not fail while building its
                         own message