Two bugs exposed during first batch ingest run after D-03 fix:
1. `.txt` files were silently skipped ("Unbekannter Dateityp") — added
plain-text read path alongside .tex and .pdf.
2. After D-03, OpenAlex resolves arXiv IDs → paper.id uses the DOI
(https://doi.org/…) but fetch_citations emits citing_id=openalex_id
(https://openalex.org/W…). FK constraint citations_citing_id_fkey
→ papers.id violated. Fix: rewrite citing_id to paper.id after
fetching from OpenAlex.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>