server : improve startup log messages (#29125)
* server-models : show source per model in log
- Show [source] tag (preset/models_dir/cache) per model instead of cryptic * marker
- Show HF hub cache path in the 'Loaded cached model presets' log
- Add hf_cache::get_cache_dir() public accessor
Assisted-by: pi:llama.cpp/Qwen3.8-27B
* cont : pad log
