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.

CLAUDE.md

@@ -12,12 +12,13 @@ provider.
 
 ## The gate — never merge or ship red
 
-The local gate is **ruff (`check` + `format --check`) + typecheck (3 tsc projects) + eslint + prettier
+The local gate is **ruff (`check` + `format --check`) + basedpyright (backend) + typecheck (3 tsc projects) + eslint + prettier
 --check + knip + vitest + pytest**. It runs on every `publish`, which **refuses to build on failure — there is deliberately no
 `--skip` flag.** Run it and get it green **before** any merge, release, or handing work to review.
 
 - Full gate (agents with the private ops dispatcher): `siftlode check`.
 - Directly, without the dispatcher: `cd frontend && npm run typecheck && npm run lint && npm run knip && npm run test`, plus backend `ruff check backend` + `ruff format --check backend` and `pytest` (the DB-lane needs Postgres).
+- **The backend type lane runs in the test image, not on the host** — `requirements.lock` cannot be installed on Windows (uvloop), and a type checker without the real packages reports nonsense. It fails on NEW errors only; the pre-existing ones sit in `backend/.basedpyright/baseline.json` and burn down epic by epic. Fixed some? Regenerate with `--writebaseline`.
 - New DB-backed test? It must actually run, not silently skip — assume `SIFTLODE_TEST_REQUIRE_DB=1`.
 
 ## Workflow