fix: pytest -m live works without extra flags

Root cause: test_schemathesis.py mutates decnet.web.auth.SECRET_KEY at
module-level import time, poisoning JWT verification for all other tests
in the same process — even when fuzz tests are deselected.

- Add pytest_ignore_collect hook in tests/api/conftest.py to skip
  collecting test_schemathesis.py unless -m fuzz is selected
- Add --dist loadscope to addopts so xdist groups by module (protects
  module-scoped fixtures in live tests)
- Remove now-unnecessary xdist_group markers from live test classes

tests/live/test_health_live.py

@@ -97,7 +97,6 @@ async def token(live_client):
 
 
 @pytest.mark.live
-@pytest.mark.xdist_group("health_live")
 class TestHealthLive:
     """Live integration tests — real DB, real Docker check, real task state."""