docs(model_loader): add comprehensive docstrings and comments; no functional changes

perf(attn): default to SDPA; gracefully fallback when flash_attn missing; use dtype arg

fix(hf): add hf_transfer for HF_HUB_ENABLE_HF_TRANSFER; remove deprecated symlink arg; add root / route

feat(api): fast FastAPI app + model loader refactor; add mock mode for tests\n\n- Add pyproject + setuptools config and console entrypoint\n- Implement enhanced field extraction + MRZ heuristics\n- Add response builder with compatibility for legacy MRZ fields\n- New preprocessing pipeline for PDFs/images\n- HF Spaces GPU: cache ENV, optional flash-attn, configurable base image\n- Add Make targets for Spaces GPU and local CPU\n- Add httpx for TestClient; tests pass in mock mode\n- Remove embedded model files and legacy app/modules