Add item bank persistence and selection helpers

[ekhanatime]

Summary

• add persistent assessment version and item bank models plus a seed script for response items and stats
• extend the blueprint engine with an item bank hydration helper, expose it through the package, and refresh documentation/ERD
• align tests with the canonical app.* imports and add coverage for the new helper

Testing

• DISABLE_REDIS_FOR_TESTS=1 DISABLE_DB_FOR_TESTS=1 PYTHONPATH=src pytest

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https://chatgpt.com/codex/tasks/task_b_68d3e074f5e0832885a107f0b7ab42e1