fix(gateway): tell mypy the speech extra may be absent

`make typecheck` failed on four missing stubs — piper, faster_whisper and numpy
twice — which made the pre-push gate red on a machine that had followed the
documented setup. `make setup` deliberately omits the speech extra; only
`make setup-speech` installs it, because faster-whisper and piper-tts pull
several GB of ML wheels for a backend the deployment does not use.
settings.tts_backend defaults to "speaches", a network call to the shared
service on 8601, and both imports are lazy inside the functions that need them.
So the absence is a runtime fact the code already handles, not a defect.

The gate was therefore failing for doing the right thing, which is how a gate
stops being read. The correct assertion is "these modules may be absent", not
"install several GB so the type checker is satisfied" — on a disk at 76%, for a
path this deployment does not take.

There was no [tool.mypy] section at all, so this adds one. numpy is listed for
the same reason as the other two: nothing depends on it directly, it arrives
with faster-whisper.

Note the packaging was already correct — speech is an optional extra and always
has been. I initially reported these as required dependencies that were missing
from the venv, having grepped for the package names and read the hits without
checking which table they sat under; `mypy>=1.11` was three lines below in the
same output, which should have said "these are extras". CLAUDE.md states it
outright. The fix is smaller than the one I first described because the repo
was already doing the right thing.

Gate now passes: secrets, ruff, mypy, 9 tests. Firmware and sim still report
undetermined, which is accurate — neither has a suite.

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

gateway/pyproject.toml

@@ -36,3 +36,28 @@ src = ["src"]
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
 testpaths = ["tests"]
+
+[tool.mypy]
+python_version = "3.11"
+
+# The speech extra is deliberately absent from a default setup. `make setup`
+# installs the gateway without it; `make setup-speech` adds faster-whisper and
+# piper-tts, which pull several GB of ML wheels for a backend the deployment
+# does not use — settings.tts_backend defaults to "speaches", a network call to
+# the shared service on 8601. Both imports are lazy, inside the functions that
+# need them, so their absence is a runtime fact rather than a defect.
+#
+# numpy is here for the same reason: nothing depends on it directly, it arrives
+# with faster-whisper.
+#
+# Without these overrides, `make typecheck` fails on a machine that followed the
+# documented setup — the pre-push gate turning red for doing the right thing,
+# which is how a gate stops being read (T-56). The right assertion is "these
+# modules may be absent", not "install several GB so the type checker is happy".
+[[tool.mypy.overrides]]
+module = [
+    "piper", "piper.*",
+    "faster_whisper", "faster_whisper.*",
+    "numpy", "numpy.*",
+]
+ignore_missing_imports = true