Preserve per-model token attribution across history derivation and reports

[sg4tech]

Summary

• Attribute usage events to explicit models or the nearest preceding Codex turn_context model.
• Add derived_model_usage for session/model-level token and cost aggregation.
• Update HTML reporting and schema freshness checks to use model-level usage.
• Document warehouse relationships and aggregation invariants.

Testing

• Added normalization and derivation coverage for Codex model switches and mixed-model totals.
• Added reporting and schema validation coverage for model-level usage.