fix(ingest): support .txt sources + fix citing_id FK violation

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>

codex/ingest.py

@@ -139,6 +139,8 @@ def ingest_paper(
 
             if suffix == ".tex":
                 text = latex_to_text(Path(source_path).read_text())
+            elif suffix == ".txt":
+                text = Path(source_path).read_text(encoding="utf-8", errors="replace")
             elif suffix == ".pdf":
                 text = pdf_to_markdown(source_path)
                 # Also extract GROBID refs
@@ -176,7 +178,10 @@ def ingest_paper(
         # ---------------------------------------------------------------
         api_citations: list[Citation]
         if paper.openalex_id:
-            api_citations = openalex.fetch_citations(paper.openalex_id)
+            raw = openalex.fetch_citations(paper.openalex_id)
+            # OpenAlex returns citing_id=openalex_id, but papers.id uses the DOI.
+            # Rewrite citing_id to paper.id so the FK constraint is satisfied.
+            api_citations = [Citation(citing_id=paper.id, cited_id=c.cited_id, context=c.context) for c in raw]
         else:
             api_citations = semanticscholar.fetch_references(paper_id)