fix(time): replace datetime.utcnow() with timezone-aware now(UTC) + asyncio test config

[RTHYMS]

Summary

Two issues surfaced by running `pytest` on Python 3.14:

1. async tests in `tests/test_basescan.py` were silently failing

4 `@pytest.mark.asyncio` tests had no pytest-asyncio plugin in dev deps and no `asyncio_mode` set, so pytest treated the coroutines as bare functions and failed them with:

```
Failed: async def functions are not natively supported.
You need to install a suitable plugin for your async framework.
```

Fix: add `pytest-asyncio` to `[project.optional-dependencies].dev` + `asyncio_mode = "auto"` under `[tool.pytest.ini_options]`.

2. `datetime.utcnow()` deprecation

`scout/registry.py` and the test_scoring `FakeActivity` factory used `datetime.utcnow()` (2 sites in production code, 1 in tests). Python 3.12 emits `DeprecationWarning` and 3.14 may remove it.

Replace with `datetime.now(timezone.utc)` — timezone-aware, identical instant.

Result

```
29 passed in 0.17s
```

(was 25 passed, 4 failed, 11 warnings)

Drive-by

CI workflow + badge.

[abhicris]

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