Updated attributes in XML

There are multiple reasons we should record when a paper (for instance) has been updated subsequent to ingestion:

- It would allow us to set `` on the site index so search engine crawlers can deprioritize pages that have not changed since the last crawl: #8642
- It would allows us to publish updates to Crossref (DOI metadata) after metadata updates: #7432

No files or tests are named in the issue; start by checking the related issues #8642 and #7432, then inspect the paper ingestion/metadata update path and XML generation flow for site index output. Locate where metadata timestamps are currently stored and emitted, and identify where a new last-modified field would be propagated into ``. Completion means updated papers carry the new updated attribute in XML and that Crossref/update consumers can use it consistently.

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