Make mypy run again, fix what it found, and add one check script

a7fb56c pinned mypy to Python 3.11 for the Pi. In a dev venv that also has the
analysis extra, numpy's stubs use 3.12 syntax, so mypy aborted on them before
checking a single file - and from the next day on four real errors went in
unnoticed (ebab03b, 0ef5a04, 9fd106b). Skip following numpy/librosa instead; only
librosa_analyzer.py touches them. Fix the four errors (variables reused with a
narrower type).

There is no CI, so scripts/check.sh is the gate: pytest, ruff, mypy, vitest, tsc
and eslint in one command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/REPO_OVERVIEW.md

@@ -110,8 +110,10 @@ reported at once.
 
 ## Notes for agents
 
-- The backend has `pytest`, `ruff` and `mypy --strict` configured in
-  `python-backend/pyproject.toml`, and no CI. Run all three before proposing changes.
+- There is no CI. `scripts/check.sh` runs every check the repo has - backend
+  `pytest`, `ruff` and `mypy --strict`, web `vitest`, `tsc` and `eslint` - and is the
+  gate: run it before proposing changes. A checker that cannot run (mypy once aborted
+  on numpy's stubs for two weeks without anyone noticing) is a failure, not a skip.
 - Prefer adding a scenario in `python-backend/scenarios/` over a hand-rolled test when
   the behaviour is end-to-end — those files are executed by the test suite.
 - `python-backend/notebooks/` is university course material on chord recognition, not