test: search-index concurrency for rebuild, queries, and lock order #320

Job	Run time
API integration tests + coverage (macos-latest)	24s
mypy	20s
Performance benchmarks (gated)	30s
Verify prod requirements.txt is sufficient	8s
API integration tests + coverage (windows-latest)	54s
pytest (macos-latest)	35s
ruff + pip-audit (ubuntu-latest)	20s
API integration tests + coverage (ubuntu-latest)	21s
Frontend unit tests (vitest) (windows-latest)	1m 8s
ruff + pip-audit (macos-latest)	21s
Frontend unit tests (vitest) (macos-latest)	31s
ruff + pip-audit (windows-latest)	1m 35s
Frontend unit tests (vitest) (ubuntu-latest)	26s
pytest (ubuntu-latest)	53s
pytest (windows-latest)	1m 34s
	10m 0s

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