Summary

• Adds models, tasks, exceptions and migration to convert XML documents to PubMed/PMC XML formats.
• Adds simplified upload-and-convert admin views (PubMed/PMC) so a single XML file can be uploaded and converted independently of other workflows.
• Adds a "Last error" column to the XML Documents list, surfacing the most recent conversion error (XPE/VDE/CDE/CHE/CPE/CTE/PME/PCE/UNE) per document.
• Fixes a double-serialization bug: generate_pubmed_for_xml_document/generate_pmc_for_xml_document were calling xml_utils.tostring(...) on strings already returned by packtools' pipeline_pubmed/pipeline_pmc, causing TypeError: Type 'str' cannot be serialized. (logged as "Unknown Error").

Test plan

• python manage.py test xml_manager — 46 tests, same 8 pre-existing unrelated failures in test_sps_package_validation.py (celery broker/test-db isolation issue), no new regressions.
• Manually verified PubMed conversion succeeds end-to-end for a real document (S1984-92302025000100304.xml) that previously failed with "Unknown Error".
• Verified upload-and-convert admin forms for PubMed and PMC via Wagtail admin (form validation, file upload, task triggering).

Issue

#122

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