Chore: Add pytest-asyncio as explicit dev dependency

[danielcopper]

Problem

pytest-asyncio is required to run async tests locally but is not listed as an explicit dev dependency in pyproject.toml or mise.toml. CI installs it via the pipeline, but local pytest runs fail with ~587 fixture setup errors if it's missing.

Fix

Add pytest-asyncio to dev dependencies so local test runs work out of the box.

Priority

P0 — developer experience, part of refactoring sprint

Area

CI, Architecture