feat: enable transcription export formats (TXT, Markdown, ALTO, PAGE XML)

[nikazzio]

Summary

Complete the transcription export pipeline. Several formats are already declared in EXPORT_FORMATS with available: False — flip them to True and implement the renderers.

Motivation

Researchers need to export their transcriptions in standard formats for publication, interoperability with other tools (Transkribus, eScriptorium, Kraken), and archival.

Formats to implement

Plain Text (.txt)

• One page per section, separated by --- or page numbers.
• Straightforward concatenation of transcription content.

Markdown (.md)

• Page headings (## Page N), transcription content, optional image references.

ALTO XML (.alto)

• Standard XML format for OCR output.
• Map transcription text to <TextLine> / <String> elements.
• Include page dimensions from IIIF canvas metadata.

PAGE XML (.page)

• Alternative OCR interchange format used by Transkribus/eScriptorium.
• Similar structure to ALTO but different schema.

Acceptance criteria

• TXT export works end-to-end via export UI
• Markdown export works end-to-end
• ALTO XML export produces valid XML against the ALTO 4.x schema
• PAGE XML export produces valid XML
• All formats available in the export dropdown
• Existing PDF/ZIP export unaffected

Technical notes

• Look at EXPORT_FORMATS dict in export service for the existing stubs.
• ALTO 4.x schema: https://www.loc.gov/standards/alto/
• PAGE XML schema: https://github.com/PRImA-Research-Lab/PAGE-XML