feat: multi-manuscript PDF merge

[nikazzio]

Summary

Allow merging multiple manuscripts (or selected pages from different manuscripts) into a single PDF document.

Motivation

Scholars often need to assemble fragments of a text that are scattered across multiple manuscripts or libraries into a single reading copy. Today they must export each manuscript separately and merge manually.

Proposed approach

• Add a "Merge PDFs" action in Library (select N manuscripts → merge).
• Allow page range selection per manuscript (e.g., ms A pages 1-10 + ms B pages 50-60).
• Reuse existing PDF export infrastructure for individual page rendering.
• Add a table of contents / bookmarks separating each manuscript section.
• Output as a single downloadable PDF.

Acceptance criteria

• Multi-select manuscripts in Library → "Merge to PDF" action
• Optional page range selection per manuscript
• Merged PDF has bookmarks/TOC for each section
• Page numbering is continuous
• Works with manuscripts that have different page sizes
• Progress indicator for large merges

Technical notes

• Use pypdf (already a dependency via PDF export) for merging.
• Alternatively, generate pages via the existing PDF pipeline and concatenate.
• Consider memory: stream pages to disk rather than holding all in RAM.
• Add a max_merge_pages config setting to prevent OOM on huge requests.