test: search-index concurrency for rebuild, queries, and lock order #318

Job	Run time
API integration tests + coverage (ubuntu-latest)	18s
Verify prod requirements.txt is sufficient	10s
ruff + pip-audit (macos-latest)	30s
pytest (ubuntu-latest)	49s
ruff + pip-audit (windows-latest)	1m 13s
Frontend unit tests (vitest) (macos-latest)	28s
Frontend unit tests (vitest) (windows-latest)	57s
ruff + pip-audit (ubuntu-latest)	24s
API integration tests + coverage (windows-latest)	46s
pytest (macos-latest)	43s
pytest (windows-latest)	1m 54s
mypy	22s
Performance benchmarks (gated)	33s
API integration tests + coverage (macos-latest)	23s
Frontend unit tests (vitest) (ubuntu-latest)	28s
	9m 58s

Provide feedback