docs: capture pdf domain language and deferred ingestion backlog

Co-authored-by: Cursor <cursoragent@cursor.com>

backlog/libreoffice-pdf-to-docx-fallback.md

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+# LibreOffice PDF→DOCX fallback
+
+## Why deferred
+
+We already convert `.doc` → `.docx` via LibreOffice. PDF→DOCX is tempting for “reuse the DOCX parser,” but styles/fonts often come out wrong — fighting the reason we chose PyMuPDF.
+
+## Trigger
+
+Heading Reconstruction on PyMuPDF has an unacceptable false-positive/false-negative rate on a representative sample, and bookmarks/fonts are absent.
+
+## Approach to try
+
+1. Optional fallback: `soffice --headless --convert-to docx` then existing `parse_docx`
+2. Compare markdown side-by-side (PyMuPDF vs converted DOCX) in Chunk Preview before making it default
+3. Keep as opt-in or auto-fallback only when PDF outline + font variance is below a threshold
+
+## Success criteria
+
+- Converted DOCX headings beat PyMuPDF heuristics on the failing sample set
+- Conversion time acceptable for upload UX (or async later — currently sync REST)