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/pyproject.toml

@@ -77,3 +77,12 @@ warn_unreachable = true
 # checkout must still type-check clean, so it needs the same treatment as librosa.
 module = ["vlc", "serial_asyncio", "ruamel.*", "librosa.*", "numpy", "feedparser"]
 ignore_missing_imports = true
+
+[[tool.mypy.overrides]]
+# Recent numpy ships stubs written in 3.12+ syntax (`type X = ...`). Checking against
+# the device's 3.11 makes mypy refuse to parse them and abort before checking anything
+# at all - which is what happened in a dev venv that has the analysis extra. Treat the
+# analysis stack as untyped instead: it is only touched by librosa_analyzer.py.
+module = ["numpy", "numpy.*", "librosa", "librosa.*"]
+follow_imports = "skip"
+follow_imports_for_stubs = true