Use Docker for local development environment

- Update README and CLAUDE.md with Docker-based dev workflow
- Add Dockerfile for containerized deployment
- Add gunicorn to requirements.txt
- Dev uses volume mount for live code reloading

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,26 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy requirements first for better caching
+COPY src/requirements.txt .
+
+# Install Python dependencies
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application code
+COPY src/ .
+
+# Create non-root user for security
+RUN useradd --create-home appuser && chown -R appuser:appuser /app
+USER appuser
+
+EXPOSE 5000
+
+# Default to production; override with --env-file at runtime
+CMD ["gunicorn", "--bind", "0.0.0.0:5000", "app:create_app()"]