perf: add mtime-invalidated LRU session cache; route APIs through it … #187

Job	Run time
API integration tests + coverage (windows-latest)	49s
pytest (ubuntu-latest)	39s
ruff + pip-audit (ubuntu-latest)	22s
API integration tests + coverage (ubuntu-latest)	18s
ruff + pip-audit (macos-latest)	17s
mypy	20s
Frontend unit tests (vitest) (macos-latest)	24s
Verify prod requirements.txt is sufficient	7s
pytest (windows-latest)	1m 29s
API integration tests + coverage (macos-latest)	19s
pytest (macos-latest)	29s
Frontend unit tests (vitest) (ubuntu-latest)	24s
Performance benchmarks (informational)	25s
ruff + pip-audit (windows-latest)	55s
Frontend unit tests (vitest) (windows-latest)	47s
	8m 4s

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