fix: resolve Permission denied for /data/reports in Coolify deployment

- Run container as root initially to fix volume mount permissions
- Use gosu to drop to appuser before starting uvicorn
- chown /data directories at startup (Coolify mounts volumes as root)

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

backend/Dockerfile

@@ -12,6 +12,7 @@ WORKDIR /app
 RUN apt-get update && apt-get install -y \
     libpq-dev \
     curl \
+    gosu \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy requirements first (Docker layer caching)
@@ -38,7 +39,6 @@ RUN chmod +x startup.sh
 # Create non-root user for security
 RUN useradd -m -u 1001 appuser && \
     chown -R appuser:appuser /app /data
-USER appuser
 
 # Expose backend port
 EXPOSE 8001