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>

python-backend/README.md

@@ -106,6 +106,8 @@ ruff check .
 mypy              # --strict, configured in pyproject.toml
 ```
 
+`../scripts/check.sh` runs these three plus the web front-end's checks in one go.
+
 `tests/test_wire.py` parses `../esp-firmware/src/Messages.h` and fails if the Python
 message ids drift from the firmware's - the contract is hand-duplicated in two
 languages, and it had already drifted once (`BUTTON_EVENT` was missing on the Python