EPUB Translate

Workflow

1. One-time setup (writes .skills-data/epub-translate/.env): scripts/epub-translate setup
2. Extract translation units: scripts/epub-translate extract --epub /path/book.epub
3. Translate via OpenAI API: scripts/epub-translate translate --job-dir <job-dir> --target-lang <bcp47> [--fraction 0.1]
4. Apply + repack: scripts/epub-translate apply --job-dir <job-dir> --translations <job-dir>/translations.jsonl --target-lang <bcp47> --out-epub /path/book.<bcp47>.epub
5. Optional: validate output: scripts/epub-translate validate --epub /path/book.<bcp47>.epub