fix: add build deps for Pillow/pycryptodome on ARM

gcc, python3-dev, libjpeg-dev, zlib1g-dev are needed to compile
Pillow and pycryptodome from source on linux/arm/v7 (no pre-built
wheels available). Build deps are purged after pip install.

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

Dockerfile

@@ -3,12 +3,17 @@
 
 FROM python:3.11-slim
 
-# Install system deps: curl (healthcheck), udev (serial), bluez+dbus (BLE)
+# Install runtime + build deps, pip install, then remove build-only packages
+# Build deps (gcc, *-dev) needed for Pillow/pycryptodome wheels on ARM
 RUN apt-get update && apt-get install -y \
     curl \
     udev \
     bluez \
     dbus \
+    gcc \
+    python3-dev \
+    libjpeg-dev \
+    zlib1g-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # Set working directory
@@ -17,8 +22,11 @@ WORKDIR /app
 # Copy requirements first for better layer caching
 COPY requirements.txt .
 
-# Install Python dependencies
-RUN pip install --no-cache-dir -r requirements.txt
+# Install Python dependencies, then remove build-only packages
+RUN pip install --no-cache-dir -r requirements.txt \
+    && apt-get purge -y gcc python3-dev libjpeg-dev zlib1g-dev \
+    && apt-get autoremove -y \
+    && rm -rf /var/lib/apt/lists/*
 
 # Copy application code
 # Note: Run 'python -m app.version freeze' before build to include version info