fix: set PYTHONPATH=/app in Dockerfile for dashboard imports

Streamlit adds the script directory to sys.path, not the working
directory, so dashboard/ wasn't importable as a package.

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

Dockerfile

@@ -7,6 +7,7 @@ RUN pip install --no-cache-dir ".[dashboard]"
 
 FROM python:3.12-slim
 WORKDIR /app
+ENV PYTHONPATH=/app
 RUN useradd --create-home appuser
 COPY --from=builder /usr/local/lib/python3.12/site-packages /usr/local/lib/python3.12/site-packages
 COPY --from=builder /usr/local/bin /usr/local/bin