perf: add mtime-invalidated LRU session cache; route APIs through it #186

Job	Run time
pytest (ubuntu-latest)	39s
ruff + pip-audit (ubuntu-latest)	25s
Performance benchmarks (informational)	28s
Frontend unit tests (vitest) (windows-latest)	47s
pytest (macos-latest)	37s
API integration tests + coverage (macos-latest)	18s
Verify prod requirements.txt is sufficient	8s
ruff + pip-audit (macos-latest)	33s
API integration tests + coverage (windows-latest)	51s
ruff + pip-audit (windows-latest)	1m 45s
API integration tests + coverage (ubuntu-latest)	24s
Frontend unit tests (vitest) (ubuntu-latest)	21s
Frontend unit tests (vitest) (macos-latest)	18s
mypy	19s
pytest (windows-latest)	1m 5s
	8m 58s

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