Add data dictionary and extend financial metrics

[usdtorero]

Summary

• add an in-app ELI5 data dictionary with ABA and accounting/finance explanations plus usability improvement ideas
• extend the financial model to include billing-lag tail months, margins, and per-role utilizations while keeping overhead out of non-operating months
• update overview metrics and summary tables to surface the new calculations

Testing

• pytest -q (fails: pandas/streamlit not installed because package download is blocked by the environment proxy)

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Codex Task