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>

docs/architecture.md

@@ -98,8 +98,9 @@ we may adopt later for streaming transcription.
 
 - **Deployed 2026-07-14**: `ghcr.io/speaches-ai/speaches:latest-cuda` on host port
   **8601**, with `Systran/faster-whisper-small` (STT) and
-  `speaches-ai/Kokoro-82M-v1.0-ONNX` (TTS, 24 kHz — the gateway resamples to the
-  16 kHz device contract; default voice `bm_george`, en-GB male). LAN-only like the
+  `speaches-ai/Kokoro-82M-v1.0-ONNX` (TTS — Kokoro is natively 24 kHz, but the
+  gateway requests the 16 kHz device contract directly via Speaches' `sample_rate`
+  extension, verified live; default voice `bm_george`, en-GB male). LAN-only like the
   Tatlock internal route — do not expose through NPM without auth. Register in
   `CONTAINERS.md`.
 - **Measured** (live round trip through the gateway code, warm): STT ~0.3 s for a