test: search-index concurrency for rebuild, queries, and lock order #317

Job	Run time
Performance benchmarks (gated)	35s
Frontend unit tests (vitest) (macos-latest)	32s
pytest (ubuntu-latest)	1m 0s
mypy	18s
ruff + pip-audit (ubuntu-latest)	15s
API integration tests + coverage (macos-latest)	18s
API integration tests + coverage (windows-latest)	50s
Frontend unit tests (vitest) (ubuntu-latest)	24s
Verify prod requirements.txt is sufficient	12s
API integration tests + coverage (ubuntu-latest)	16s
Frontend unit tests (vitest) (windows-latest)	53s
pytest (windows-latest)	1m 33s
ruff + pip-audit (windows-latest)	43s
pytest (macos-latest)	38s
ruff + pip-audit (macos-latest)	13s
	8m 40s

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