ci: type-check the backend — the gate's ninth lane (E27 S3)
Three tsc projects held the frontend and NOTHING held the backend: ~36k lines of `app/`, annotated in 113 of 128 files, that no lane had ever read. Asymmetric by accident, not by decision. basedpyright, in basic mode. Measured against mypy on this tree with the dependencies actually installed — without them basedpyright errors on every import and mypy's --ignore-missing-imports quietly turns them into Any and UNDER-reports. Both land ~190-200 errors with the same distribution, so the choice fell to what differs: basedpyright is ~2x faster (4.4s vs 7.4s, and this runs on every `siftlode lint`), resolves our `app.*` imports from its own config with no MYPYPATH, and has a first-class `--writebaseline`. That last one is the ratchet itself; mypy would have needed a third-party tool for it. 198 pre-existing errors are baselined, so the lane fails on NEW errors only and the list burns down epic by epic. A green lane means no new damage, not a clean codebase. It runs in the pytest test image rather than on the host, and that is forced rather than chosen: `requirements.lock` cannot be installed on Windows at all — uvloop refuses to build there — so a host venv could only ever be the lock minus a package, and a type checker without the real third-party packages reports nonsense rather than nothing. The image installs the pin from requirements-dev.txt, so the lane's version IS the pin and there is no host copy to drift. The cost is that `siftlode lint` now needs Docker, which it did not before.
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