fix: warn when aiograpi setup lacks dependency

[subzeroid]

Summary

• detect missing optional aiograpi dependency during interactive and non-interactive setup
• print the same pipx/uv install guidance used by backend startup failures
• isolate aiograpi/INSTO_BACKEND env vars in CLI tests

Verification

• uv run ruff check
• uv run ruff format --check
• uv run mypy insto
• uv run pytest tests/test_cli.py tests/test_config.py tests/test_backend_contract.py -q
• uv run pytest --cov=insto --cov-fail-under=75