Add CI workflow to run pytest on PRs

[bradleesand]

Summary

• Adds .github/workflows/tests.yml running the full pytest suite on Python 3.11
• Triggers on every PR, on pushes to main, and via workflow_dispatch
• Installs runtime deps from requirements.txt plus pytest, pytest-asyncio, and httpx (needed by FastAPI TestClient)

Why

There is currently no general CI workflow that runs the test suite — only the targeted music-search-benchmark and release workflows. Adding a pytest CI run on every PR will catch regressions and surface the current state of the suite.

Notes

• This intentionally does not fix existing test failures. Several tests in tests/test_*api*.py, tests/test_intent_*, etc. currently fail because of a stale test helper that clears app.on_startup but does not re-set app.state.config_path (a fix exists on arr_integration but is part of a larger refactor). Other unrelated failures exist as well. Surfacing these in CI is the first step toward fixing them.

Test plan

• Workflow appears on the PR and runs to completion (pass or fail)
• Pytest output is visible in the action logs
• Subsequent PRs trigger the workflow automatically