fix(ml): degrade to statistical features on any extractor failure, implement download-models

[Liohtml]

Summary

extract_features only caught ImportError ("torch not installed"), but the ResNet18 path also downloads ImageNet weights from download.pytorch.org on first use. In an air-gapped environment — the deployment the "on-device" pitch targets — that download raises URLError/RuntimeError and crashed the whole ml_analysis step instead of falling back.

Fixes #137

Changes

• src/medcheck/pipeline/ml_analysis.py: split into _resnet_features / _statistical_features; extract_features now degrades to the statistical fallback on any extractor failure, logging the reason
• src/medcheck/main.py: implement the download-models stub — builds the feature extractor once to pre-cache the weights (clear error + exit 1 when torch is missing or the download fails)
• README.md: document the one-time model download and the medcheck download-models pre-fetch for offline environments (previous wording implied no network access at all)
• Tests: 2 new extract_features fallback tests (RuntimeError download failure, ImportError), 2 new CLI tests for download-models

Testing

• Existing tests pass (uv run pytest) — 184 passed
• New tests added for new functionality — 4 new tests
• Coverage does not decrease (uv run pytest --cov-fail-under=85) — 88.28%
• Linting passes (uv run ruff check .)
• Type checking passes (uv run mypy src/medcheck --strict)
• Pre-commit hooks pass — ruff hooks pass locally; full pre-commit not run

🤖 Generated with Claude Code

https://claude.ai/code/session_01W4svt5QTs4WUMSy4HwiVt9

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 0336537 and bfb5bb8.

📒 Files selected for processing (5)

• README.md
• src/medcheck/main.py
• src/medcheck/pipeline/ml_analysis.py
• tests/unit/test_cli.py
• tests/unit/test_pipeline/test_ml_analysis.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/dependabot-prs-q6u94z

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[codecov]

Codecov Report

❌ Patch coverage is 92.10526% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/medcheck/main.py	75.00%	3 Missing ⚠️

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