test: search-index concurrency for rebuild, queries, and lock order #319

Job	Run time
mypy	25s
Performance benchmarks (gated)	29s
Verify prod requirements.txt is sufficient	11s
Frontend unit tests (vitest) (macos-latest)	44s
ruff + pip-audit (windows-latest)	1m 12s
ruff + pip-audit (ubuntu-latest)	20s
ruff + pip-audit (macos-latest)	37s
API integration tests + coverage (windows-latest)	55s
pytest (macos-latest)	39s
Frontend unit tests (vitest) (windows-latest)	1m 19s
pytest (windows-latest)	1m 35s
Frontend unit tests (vitest) (ubuntu-latest)	24s
API integration tests + coverage (macos-latest)	20s
API integration tests + coverage (ubuntu-latest)	21s
pytest (ubuntu-latest)	52s
	10m 23s

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