refactor(epub): make epub extraction / creation code reusable

[imnotjames]

Description

The epub metadata writer has a function that creates an epub from a directory, to extract an epub to a directory using archive service, and a helper to find the OPF path. All three of these are useful in other places, so this extracts them to the archive service, epub content reader, and a new epub content writer utility class.

Linked Issue

related to #2001

Changes

• Adds ArchiveService.extractToDirectory helper
• Adds EpubContentReader.findOPFInExtractedEpub
• Adds EpubContentWriter.createEpubFromDirectory
• Refactors EpubMetadataWriter to use these helpers
• Refactors CBXConversionService to use helper extract helper

Manual Testing Steps

Screenshots (Optional)

Additional Context (Optional)

AI Disclosure

Checklist

• This PR links and implements an accepted issue.
• This PR is a single focused change.
• There are new or updated tests validating this change.
• I ran just ui check and just api check.
• I have added screenshots if there were any UI changes.
• I have disclosed any AI usage as per the organization AI Policy above.
• I understand all of my submitted changes.

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