Add EPUB embedding support

[HMAKT99]

AKF currently embeds into DOCX, PDF, HTML, images, and other formats. EPUB is a natural fit since it's essentially a ZIP of HTML/XML files.

What to do:

• Add an EPUB handler to python/akf/universal.py
• Embed AKF metadata as an OPF <meta> element or as a JSON-LD script in the content documents
• Add extract support to read it back
• Add tests

Relevant files:

• python/akf/universal.py — format embedding/extraction logic
• python/tests/test_universal.py — tests

Helpful context:

• Look at the HTML embedding handler for reference (JSON-LD approach)
• EPUB 3 supports custom metadata in the OPF package document