parakeet: rewrite on sherpa-onnx; own the wrapper end-to-end

The Shadowfita FastAPI wrapper hit two unfixed upstream bugs on the
first real /transcribe call — chunker return-shape mismatch (open
issue #16) and a `torchaudio.tensor` that doesn't exist (open #10).
Rather than babysit someone else's half-tested code, switched to
sherpa-onnx with the prebuilt int8 Parakeet-TDT tarball from k2-fsa,
and wrote our own ~60-line FastAPI wrapper.

Moving parts now owned in-tree:
  Dockerfile      CUDA 12.8 + cuDNN 9 runtime base, installs
                  sherpa-onnx==1.12.39+cuda12.cudnn9 + fastapi +
                  soundfile + libasound2 (sherpa-onnx links to ALSA
                  at load time even when we never touch a mic).
  app.py          OfflineRecognizer.from_transducer() once at startup;
                  /transcribe and /v1/audio/transcriptions both accept
                  multipart uploads and return {"text": ...}.
  entrypoint.sh   Idempotent model download to /models on first run
                  (~400 MB int8 tarball), then exec uvicorn.

Smoke test: 0.wav (bundled in the tarball, The House of the Seven
Gables excerpt) transcribes cleanly in ~1.2s on GPU.

PARAKEET_MODEL_URL in .env lets you swap to the v3 (25-language)
tarball without touching any other files. Wipe *.onnx + tokens.txt
from the models dir and the entrypoint re-downloads.

stacks/parakeet/Dockerfile

@@ -0,0 +1,41 @@
+# syntax=docker/dockerfile:1.6
+#
+# Parakeet-TDT ASR via sherpa-onnx (ONNX Runtime + CUDA).
+#
+# We own this whole image — not forked from an upstream wrapper. ~70 MB of
+# application layer over the CUDA+cuDNN runtime base.
+
+ARG CUDA_BASE=nvidia/cuda:12.8.0-cudnn-runtime-ubuntu22.04
+FROM ${CUDA_BASE}
+
+ENV DEBIAN_FRONTEND=noninteractive \
+    PIP_ROOT_USER_ACTION=ignore \
+    PYTHONUNBUFFERED=1 \
+    PATH="/opt/venv/bin:${PATH}"
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends \
+        python3 python3-pip python3-venv \
+        libsndfile1 \
+        libasound2 \
+        ca-certificates wget bzip2 \
+ && rm -rf /var/lib/apt/lists/* \
+ && python3 -m venv /opt/venv
+
+RUN pip install --no-cache-dir \
+        fastapi \
+        'uvicorn[standard]' \
+        python-multipart \
+        soundfile \
+        numpy \
+ && pip install --no-cache-dir \
+        sherpa-onnx==1.12.39+cuda12.cudnn9 \
+        -f https://k2-fsa.github.io/sherpa/onnx/cuda.html
+
+WORKDIR /app
+COPY app.py /app/app.py
+COPY entrypoint.sh /usr/local/bin/entrypoint.sh
+RUN chmod +x /usr/local/bin/entrypoint.sh
+
+EXPOSE 8000
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]