fix: index Word, PowerPoint and text files that never got a PDF rendition

The passage indexer read every document through its PDF rendition, falling back
to the original file when there was none - but then still parsed those original
bytes as a PDF. A Word file whose LibreOffice conversion had failed at upload
therefore failed to index with "Invalid PDF structure". Found while indexing
existing documents: three real .docx letters failed this way.

versionText now reads the PDF rendition when one exists (it carries page
numbers), and otherwise reads the original with a reader that suits its type -
mammoth for Word, the slide reader for PowerPoint, the sheet reader for a
spreadsheet, plain text for a text file - and never parses non-PDF bytes as a
PDF. A Word file with no rendition indexes without page numbers, which is
correct: a Word document reflows and has no fixed pages.

Verified on the live server: the three failed letters now index (5, 3 and 5
passages of clean text), and the full backfill finished with 52 documents and
412 passages, 0 failures.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PoCRDAPyEq4wwBVFFbDzck