Feature: Export annotations as EPUB

[texasbe2trill]

Summary

KoNotes supports exporting to Markdown, JSON, HTML, CSV, and plain text. An EPUB export would let users read their annotations as an e-book on any reader — a natural fit for a tool built around reading.

Requirements

• Create services/export_epub.py
• Generate a valid EPUB 3 file with one chapter per book
• Include highlights, notes, and bookmarks grouped by chapter
• Add book metadata (title, author) to EPUB metadata fields
• Register in CLI (konotes export <file> -f epub)
• Add tests in tests/test_export_epub.py

Considerations

• Use a lightweight EPUB library (e.g., ebooklib) or generate the ZIP/XML structure directly to minimize dependencies
• Could be an optional dependency like pip install '.[epub]'
• Follow the existing export pattern in services/export_csv.py or services/export_markdown.py