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>

scripts/check.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# Every check this repo has, in one command. There is no CI, so this is the gate:
+# run it before committing, and treat a checker that cannot run as a failure.
+#
+#   scripts/check.sh            backend and web
+#   scripts/check.sh backend    just one half
+#   scripts/check.sh web
+#
+# PYTHON picks the interpreter (default: python-backend/.venv/bin/python, else python3).
+
+set -euo pipefail
+
+repo="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+what="${1:-all}"
+
+backend() {
+    cd "$repo/python-backend"
+    local python="${PYTHON:-}"
+    if [[ -z "$python" ]]; then
+        if [[ -x .venv/bin/python ]]; then python=.venv/bin/python; else python=python3; fi
+    fi
+    echo "== backend: pytest"
+    "$python" -m pytest -q
+    echo "== backend: ruff"
+    "$python" -m ruff check .
+    echo "== backend: mypy"
+    "$python" -m mypy
+}
+
+web() {
+    cd "$repo/web"
+    echo "== web: vitest"
+    npm run --silent test
+    echo "== web: tsc + eslint"
+    npm run --silent check
+}
+
+case "$what" in
+    all) backend; web ;;
+    backend) backend ;;
+    web) web ;;
+    *) echo "usage: $0 [all|backend|web]" >&2; exit 2 ;;
+esac
+echo "== all checks passed"