fix: make /app/.assets writable for model cache (mkdir + chown before USER)

Dockerfile

@@ -106,6 +106,9 @@ COPY --chown=user:user DeepFakeAI/ /app/DeepFakeAI/
 RUN mkdir -p /app/uploads/source_images /app/uploads/target_videos /app/uploads/result_videos && \
     chown -R user:user /app/uploads
 
+# Ensure model cache directory is writable for runtime model downloads
+RUN mkdir -p /app/.assets/models && chown -R user:user /app/.assets
+
 # Switch to user (upload directories already created with proper permissions)
 USER user
 ENV PATH="/home/user/.local/bin:$PATH"