[codex] expand JSON progress export

[saurabhhhcodes]

Summary

• expand the JSON backup export with profile metadata and platform usernames
• include richer per-question progress fields such as question id, difficulty, and tags when present
• keep the export free of secrets while preserving the existing download flow

Closes #278

Validation

• python3 -m pytest tests/test_progress_import.py -q
• git diff --check

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