feat: Add initial backend application and update Dockerfile for its setup.

Dockerfile

@@ -34,16 +34,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 # Create non-root user
 RUN useradd --create-home --shell /bin/bash foxy
 WORKDIR /app
+RUN chown foxy:foxy /app
 
 # Python dependencies
-COPY backend/requirements.txt ./
+COPY --chown=foxy:foxy backend/requirements.txt ./
 RUN pip install --no-cache-dir -r requirements.txt
 
 # Backend source
-COPY backend/app ./app
+COPY --chown=foxy:foxy backend/app ./app
 
 # Frontend static files (served by FastAPI)
-COPY --from=frontend-build /build/dist ./static
+COPY --chown=foxy:foxy --from=frontend-build /build/dist ./static
 
 # Healthcheck
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \