Drop audioop: request 16 kHz directly via Speaches sample_rate extension

Settles the Python version question: floor >=3.11, no ceiling.
Container moves to python:3.13-slim, CI tests on 3.13. The embedded
Piper fallback resamples with numpy (already present via [speech]).
Verified with a live round trip at 16 kHz.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
 
       - uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version: '3.13'
 
       - name: Install
         run: pip install -e "./gateway[dev]"