Issue 732 guard negative lookback days

[Kabukabs]

Summary

What does this PR change and why?

Type of change

fixes #732
closes #732

• Bug fix
• New feature
• Data/schema change (affects feature columns, data models, Avro schema)
• Model change (affects training, inference, or evaluation metrics)
• Infrastructure / CI change
• Documentation only
• Security fix

Changelog

### Added / Changed / Fixed / ...
- <your entry here>

Impact assessment

Data / feature schema changes

• No schema changes
• New feature columns added (update data/feature_dictionary.md)
• Feature columns removed or renamed (update feature_schema_hash; note downstream impact)
• Avro schema evolved (backward-compatible / breaking)

Model changes

• No model changes
• Model retrained — include before/after AUC-ROC / F1 in the summary above
• Inference API changed — update ledgerlens-core if RiskScore shape changed
• Feature schema hash will change — all consumers need to reload models

Downstream impact

• ledgerlens-api — REST API response shape may change
• ledgerlens-dashboard — visualization or SHAP field names may change
• ledgerlens-contract — on-chain RiskScore struct may change
• ledgerlens-core — shared types or thresholds may change
• None

Checklist

• make test passes locally
• make lint and make format are clean
• New/changed behavior has test coverage
• CHANGELOG.md updated under [Unreleased]
• If a shared contract changed (RiskScore, asset pair format, feature
  schema), linked issues/PRs in ledgerlens-core and downstream repos