fix: make pytest 9 dependency set solvable

[saagpatel]

Summary

• bump pytest to the Dependabot-requested 9.x range
• bump pytest-asyncio to the compatible 1.x range so dependency resolution succeeds
• supersedes Dependabot PR chore(deps): update pytest requirement from <9.0,>=8.0 to >=9.1.1,<10.0 in /backend #67, which fails because pytest-asyncio>=0.24,<1.0 requires pytest<9

Verification

• uv venv --python 3.12
• uv pip install -r requirements.txt
• uv pip install ruff mypy
• uv run ruff check .
• uv run pytest (378 passed)
• uv pip check
• git diff --check

Note: uv run mypy app still reports the repo's pre-existing strict typing backlog; CI currently marks that step continue-on-error.