fix: add gcc/build-essential for webrtcvad, drop silero-vad pip install

resemblyzer depends on webrtcvad which compiles C extensions — gcc and
build-essential are required in slim containers. silero-vad pip package
has a broken native .so in python:3.12-slim; VAD model is now loaded at
runtime via torch.hub.load() which downloads on first use.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cron/Dockerfile

@@ -1,10 +1,12 @@
 FROM python:3.12-slim
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    libsndfile1 && rm -rf /var/lib/apt/lists/*
+    libsndfile1 gcc build-essential && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
 COPY pyproject.toml .
+# Install torch CPU-only, resemblyzer, and cron deps
+# silero-vad pip package excluded — not needed in cron (no VAD processing)
 RUN pip install --no-cache-dir torch --index-url https://download.pytorch.org/whl/cpu && \
     pip install --no-cache-dir resemblyzer scipy numpy httpx pydantic-settings