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/test_schemathesis.py

@@ -10,6 +10,7 @@ replace the checks list with the default (remove the argument) for full complian
 
 Requires DECNET_DEVELOPER=true (set in tests/conftest.py) to expose /openapi.json.
 """
+import pytest
 import schemathesis
 from hypothesis import settings
 from schemathesis.checks import not_a_server_error
@@ -18,6 +19,7 @@ from decnet.web.api import app
 schema = schemathesis.openapi.from_asgi("/openapi.json", app)
 
 
+@pytest.mark.fuzz
 @schemathesis.pytest.parametrize(api=schema)
 @settings(max_examples=5, deadline=None)
 def test_schema_compliance(case):