feat: CSV/spreadsheet metadata export from Library

[nikazzio]

Summary

Allow exporting the Library catalog as a CSV or Excel file containing manuscript metadata.

Motivation

Researchers need to produce inventories, share lists with colleagues, or import data into reference managers (Zotero, EndNote). A CSV export of the library catalog is a standard requirement.

Proposed fields

• Manifest ID / URL
• Title
• Provider / Institution
• Date (if available in metadata)
• Page count
• Transcription status (% complete)
• Download date
• Local path
• Custom tags/notes (if implemented)

Acceptance criteria

• "Export as CSV" button in Library toolbar
• CSV includes all key metadata fields
• UTF-8 encoding with BOM for Excel compatibility
• Optional: Excel (.xlsx) format via openpyxl
• Filters applied in Library UI are reflected in export (export what you see)

Technical notes

• Python csv module for CSV, openpyxl for Excel (optional dependency).
• Route: GET /library/export?format=csv
• Stream response for large libraries.