feat: add DjVu conversion support

[al-n]

Summary

• install DjVuLibre's ddjvu utility in the production image
• add DjVu-to-PDF and DjVu-to-TIFF conversion through ddjvu
• register and document the new converter
• add unit coverage for converter metadata, command arguments, logging, and failures

Notes

ddjvu renders each page into the resulting PDF or TIFF. This supports scanned/image-only DjVu documents that Calibre cannot convert, but it does not preserve an embedded searchable text layer.

Testing

• bun test — 107 passed, 8 skipped, 0 failed
• tsc --noEmit
• knip
• Prettier and ESLint checks for the changed TypeScript files

Closes #583

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Summary by cubic

Adds DjVu-to-PDF and DjVu-to-TIFF conversion via ddjvu, so DjVu inputs that previously failed now convert. Output is rasterized; embedded searchable text is not preserved.

• Installs djvulibre-bin.
• Registers a dedicated djvu converter for .djvu/.djv and restores its priority ahead of generic document converters.
• Runs ddjvu -format=<pdf|tiff> and logs stdout/stderr.
• Updates README and adds unit tests.

Rollout

• No API changes.
• Local development must install djvulibre-bin or use the updated image.

^{Written for commit b9f49f0. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 5 files

_{Re-trigger cubic}

[Copilot]

Pull request overview

Adds first-class DjVu conversion support by installing DjVuLibre tooling in the production image, registering a new DjVu converter, and adding unit tests + documentation so DjVu inputs can be converted to PDF/TIFF via ddjvu.

Changes:

• Install djvulibre-bin (for ddjvu) in the Docker release image.
• Add a new djvu converter and register it in the converter dispatch table.
• Add unit tests for converter metadata/arguments and document the converter in the README.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Dockerfile	Installs djvulibre-bin to provide ddjvu in the production image.
src/converters/djvu.ts	Introduces ddjvu-based DjVu→PDF/TIFF conversion implementation.
src/converters/main.ts	Registers the new converter in the global dispatch table.
tests/converters/djvu.test.ts	Adds unit tests for DjVu converter properties and invocation args (plus common converter behaviors).
README.md	Documents DjVuLibre as a supported converter and updates counts.

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[cubic-dev-ai]

All reported issues were addressed across 4 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

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