fix(export): stop leaking internal description in public archive export

[level09]

Problem

The public archive export (`flask export public`, added in #345) serialized both the internal `description` and the SJAC-authored `public_description` into `documents.json`. The internal `description` holds staff notes that are not meant for public consumption, so it should never leave Bayanat.

Fix

Drop `description` from `serialize_bulletin()`. Only `public_description` is exported now.

Notes

• The archive import side reads the field with `doc.get("description")`, so a missing key does not break import (it just stores NULL).
• Follow-ups for a separate pass: the archive's `EXPORT_SCHEMA.md` still lists `description` as required and the archive schema/FTS still index it. Those should be reconciled, and redaction of `public_description` itself is tracked separately.

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