feat: Add Upload-Post integration for cross-posting to TikTok/Instagram

- Add upload_post.py service for Upload-Post API integration
- Add upload_post configuration options to config.example.toml
- Integrate cross-posting in task.py after video generation
- Add documentation in docs/upload-post.md (English & Chinese)

Upload-Post (https://upload-post.com) allows cross-posting videos
to TikTok and Instagram with a single API call. This integration
adds automatic cross-posting after video generation when enabled.