refactor: centralize source API requests, polling, and path parsing
- Add request_source_response and request_source_json helpers with 429 retry logic
- Introduce shared split_path_segments and parse_provider_model_url utilities
- Migrate CivitAI, CivArchive, HuggingFace, and ModelList sources to centralized helpers
- Add generic pollBackgroundTask utility for frontend progress polling
- Add tests for request_source_json, split_path_segments, parse_provider_model_url
- Add huggingface-hub to requirements