test: fix async fixture isolation, add fuzz marks, parallelize with xdist

- Rebuild repo.engine and repo.session_factory per-test using unique
  in-memory SQLite URIs — fixes KeyError: 'access_token' caused by
  stale session_factory pointing at production DB
- Add @pytest.mark.fuzz to all Hypothesis and Schemathesis tests;
  default run excludes them (addopts = -m 'not fuzz')
- Add missing fuzz tests to bounty, fleet, histogram, and repository
- Use tmp_path for state file in patch_state_file/mock_state_file to
  eliminate file-path race conditions under xdist parallelism
- Set default addopts: -v -q -x -n logical (26 tests in ~7s)

tests/api/stats/test_get_stats.py

@@ -30,6 +30,7 @@ async def test_stats_includes_deployed_count(mock_state_file, client: httpx.Asyn
     assert "deployed_deckies" in _data
     assert _data["deployed_deckies"] == 2
 
+@pytest.mark.fuzz
 @pytest.mark.anyio
 @settings(**_FUZZ_SETTINGS)
 @given(